It's not 'nonsense'... but nor is it the whole story.
Potty training means:
- knowing when they need to go before they're going.
- the physical control to hold it
- the ability to let someone know they need to go
- the desire to go in the potty/toilet
- being habituated to the feeling of going on the toilet/potty/NOT in a nappy/bed/etc
- for those who used nappies - removing that habituation, replacing it with a new habit.
That first stage takes some time, whether its humans or dogs (no puppy is housetrained in 2 weeks btw!)
Currently she'll be at the stage where her child is aware of the need to go, she is aware of her childs toilet schedule and the physical signs of needing to go.
As her child is NOT habituated to the sensation of going in a nappy or in clothing, it will be easier for her to see the signs that her child wants to go.
This doesn't mean her child can hold it as long as an adult, that's physically impossible, nor can he take himself to the toilet, he's too young for that, nor is it likely he could ask anyone BUT his parents/nanny to sort him out because she and presumably her husband and nanny are the only ones who know what his physical cues are.
But it doesn't mean its bollocks or bullshit either - it's just a different way of doing it.
Anyone toilet training using nappies IS habituating their child to the feeling of going in nappies and that means there is no need to be fully aware of bowel/bladder fullness/urge at this point.
So when you do come to change things round, you'll need to break that habit (and try changing your own toilet habits, that isn't easy!) AND build a new habit, alongside teaching that there is now a need for bladder/bowel awareness.
Which isn't easy, and is why toilet training can take some time for some kids.
But then her method means you do need to be with your child AND be super vigilant all the time, it definitely wouldn't work if your child was in nursery or you're splitting your attention between child and WFH.