It's obviously not true in our current context because around 80% of women start off breastfeeding, and of those, around a third experience no problems, a third experience problems but are able to overcome them (with or without support) and continue breastfeeding until a time they wanted to stop, and a third are not able to overcome them, and stop before they had wished to. It's that last third that the big question mark hangs over. (Some of the 20% who don't start BF at all will be second+ time mums who fitted into this last third previously and, fairly, want to spare themselves the pain.)
I don't think I have ever come across a single person (that I didn't think was trolling) who thinks that this third "did not try hard enough" or that the issue is effort or willpower related.
It's likely that some portion of this group have physical, hormonal, biological issues that no amount of support could mitigate. The 97% figure suggests this is only 3% though, rather than the ~33% accounted for by statistics.
Is 3% accurate? I think it's more complicated than that. Certainly 3% as a minimum isn't unbelievable, because evolution isn't some super-optimised thing and nature does include errors. I think it's a bit low personally. I think it's more likely that the figure of women who are always biologically going to struggle to produce milk is probably closer to 10%, partly because in a survival sense, having 90% of women able to produce milk means there would always be a nursing mother around even if it was not the baby's biological mother, and partly because more up to date research shows links between conditions like PCOS and breastfeeding difficulties, so it's also possible that if we go way back, mothers who would have struggled to breastfeed might also have struggled to conceive. Plus, there are environmental factors - various sources of hormone disruption - that would not have existed thousands of years ago.
But I do think the number who have a biological/medical reason to have difficulties breastfeeding is lower than the number of women who do experience insurmountable difficulties breastfeeding, and that's because I think our culture subtly undermines breastfeeding success a lot of the time and places barriers in the way of a breastfeeding relationship developing well. It's not necessarily overt and it's not necessarily malicious in any way.
I also think the middle third (who experience problems but overcome them) are likely to be increased by these cultural barriers.
When I say cultural barriers I mean things like this. Most new mothers have never seen any baby being breastfed, or have only seen it briefly, or many years ago, because we have fewer babies, we spend less time around babies in general, babies are less often breastfed and for less long, and it is seen as something taboo or embarrassing so it is polite to look away. When you see something frequently then you unconsciously pick up knowledge about what is normal and how to do it.
For example, when I went to work in a shop as a teenager, I automatically adopted the same way I had heard other shop assistants say things like "Would you like a bag?" "Here's your change. Have a nice day." "That's £3.72 please." When I first drove a car, although I needed lessons to really understand what to do, I had a basic understanding that I would need to turn the wheel to steer, to press the pedals with my feet, to change gear, to indicate, etc. It actually felt a lot more natural than I was expecting it to. I know what traffic light signals mean, even if I have to learn the exact meaning of the amber light for example. If I was to go and train as a train driver, I would probably need much more initial training on how to use the basic controls, because I have not spent a significant amount of time sitting next to a train driver observing them. I would have to study the signals to remember which signal means what because I have not already got any preexisting knowledge about train signals.
The same thing happens with anything we learn, including breastfeeding. In fact though, the thing we tend to have experience of is bottle feeding. We might even have bottle fed a baby before we have our own, and a lot of this observed knowledge tends to intrude when we try to breastfeed. For example, with a bottle you can look at the markings on the side to understand how much the baby has drunk and therefore whether you ought to try to convince them to take a little bit more or whether you are happy that they have had enough. Knowing how much they have drunk leads you to make assumptions about how often it should be before they are fed again. This feels like obviously important information and so the absence of it with breastfeeding can feel quite worrying. But in reality you don't need to know this at all, because babies (just like humans of any age) don't necessarily eat exactly the same measured amounts at every meal or even every day. The way that bottle feeding works means that it helps to be able to predict how much the baby will want to drink and when, so you don't waste formula. A bottle also has a set amount in it which when it is done, it is done. And then it takes time or effort to refill. But breastfeeding doesn't work this way. Essentially you have an unlimited supply of milk (not really, but in terms of the amount a small baby will drink) and it doesn't matter how frequently they feed, they can usually get more if they need it because your body just makes more on demand. The best way to increase milk supply is to feed more, because that will send a feedback message to your hormones to increase speed of production. But if you're unconsciously making an assumption that the breast is like a bottle which has a set amount and then needs time to refill, that can cause you to behave in a way which doesn't send those messages to the body and the body continues producing milk at the slower rate, or even cause milk to pool in the milk ducts, which is a backup system to account for excess supply, which is common after growth spurts or early on in breastfeeding when your supply is hormonally driven because your body doesn't know if you had twins or not. Actually if you're thinking about bottles, then you might even feel like you should be waiting for this time when your breasts feel "full", which is counterproductive as too much of this milk pooling sends the opposite message (slow down production/conserve energy).
I couldn't go into every possible way which bottle feeding as a norm or unfamiliarity with breastfeeding causes problems because it will be too long, but essentially if you look at any Attachment Parenting stuff (the older stuff like Sears, not the insane modern stuff like Komisar) which emphasises maximising close contact between the baby and breastfeeding mother, unrestricted access to the breast particularly during sleep, lots of skin to skin or other close body contact, all of this is great for increasing success of breastfeeding and our society is not at all set up for most of it, as well as delivering messages suggesting that "feeds" are a set, discrete thing which happen X times a day and it would be beneficial to measure them. Many practices which are actively normal or encouraged or even a genuinely good thing do work against breastfeeding as well. And there will be areas where success of BF is directly in competition with other things (such as SIDS prevention for example). So it's not simple at all. And lack of access to good, knowledgable, experienced BF support is an issue.
I would highly recommend The Politics of Breastfeeding as an interesting read on the topic.