I don't think it's an important shift, or much of a shift, and I'm not fond of this kind of framing.
I don't think strengthening the language from 'autism is mostly genetic predisposition' to 'autism is genetic' is going to stop people blaming the parents and/or autistic people, even when it's ourselves, even with the qualifiers that the National Autism Society used that includes most, often, and usually. It's a solid very simple introduction to make autistic people come across in the best light, as is their job, it's not shifting anything.
It's not going to stop people looking for something to blame or "solve" autism. All we need to do is look at pretty much any other 'born with it' condition to see that knowing that has never led to more understanding, support, or effort to make society work for us. Knowledge of a condition, whether present at birth or acquired later, has never led to that on its own. It's been through disabled people fighting for it - I mean, we didn't get more accessible spaces through people understanding mobility disabilities, it came from disabled people chaining themselves to buses and crawling up stairs in groups.
I also think, as others have mentioned, that we know other conditions that are not genetic can also appear very much like and/or are highly co-morbid with autism. We currently have an issue where some conditions are being underdiagnosed because an autism diagnosis is treated as the answer, leaving people misunderstood and not getting the needed support. I also think we are not understanding any disability if we're not looking at how environment shapes how it presents and impacts how disabling a condition can be.
There is also another issue, the part that makes me not fond of this framing.
Alcohol and other drug exposure in utero have been shown significant environmental factor. Large systemic research reviews give strong evidence that a person with FASD is at least twice as likely to be autistic as the general population. The National Autism Society chose to leave that environmental factor out. It doesn't fit the branding. There is no push for research on how having FASD or similar exposure related condition impacts further generations.
To be blunt - it doesn't fit the ableist cultural ideal that a disability only deserves understanding, supporting, compassion, for adjustments in society when they are obviously happenstance, when we are blameless.
I was born with multiple disabilities, and acquired a couple more along the way. People's attitudes often depending on how visible they are, how much that person think I'm "overcoming" being disabled (this part I think is getting worse over the years), and/or who they feel like blaming at that time - it rarely comes down understanding in any significant degree, especially not in understanding the whys behind my conditions. I'm also married to a man who became physically disabled after an assault that damaged his spine among other important parts, and had to deal with medical professionals outright telling him that they had thought he wasn't worth treating because they thought his injuries were from a motorcycle accident because of how he looks. He's dealt with a lot of shite because of people attributed the blame of his disabilities to him often pretty readily.
I don't care if my or my kids' autism is entirely genetic or a genetic predisposition shaped by environment. I do care how about the cultural issues that pits disabilities against each other and treats a condition having avoidable environmental factors as if it makes the person with it less worthwhile. The whole idea that being 'born with it' somehow makes it better, somehow now means we can shift to understanding and support, is to fall into ableist cultural ideal that such things come after figuring out blame. I'm used to it working that way, I understand why a charity promoting greater acceptance of autism is doing what the NAS has done within the cultures we have by pushing forward the stronger 'blameless' argument, but it's still a bit sad - it's not understanding or support, and it's not adjusting society, it's fitting our description in a way society will find more acceptable, a certainty - with qualifiers - that is more palatable. That's how life with disabilities tends to work, making ourselves more palatable until we get to the point where it's obviously not working and we push back. I'm not sure what that would look like when it comes to autism or similar conditions.