When you are being told "happy son Vs dead daughter" rational decisions possibly go out the window. It was (is?) sold as a fait accompli that if the child wasn't allowed to transition they would kill themselves, must be very difficult to resist under tat level of pressure - especially when the so called experts are telling you this.
When you have a desperately unhappy child, you are frantic for experts to advise you on how to make them better, happier, able to cope. The terror of losing a child to suicide is unspeakable - sadly, it's something many parents with autistic kids know all too well already. The parent is therefore incredibly vulnerable to being sold a narrative that X will be a magic cure, and when the child is equally emphatic, of course you go with that. And if a parent has gone down the full intervention route, they're going to have a huge emotional cost to any backpedal.
People go nuts over whether other people use purees or baby led weaning, when the choice isn't their own - we've all seen those threads when in those stages, ourselves. The fear of not having parenting validated comes, I think, from the fear of failing or harming your child. My heart absolutely goes out to parents dealing with this one and trying to do the very best for their kids. They, in many cases, have been horribly failed as well, I think. Just not as badly as the parents who were kept out of the picture completely until they found their kids had already been socially transitioned at school with blind affirmation - no watchful waiting, no considered approach at all.
Any movement that advises parents be kept out of the loop unless guaranteed to be cheerleaders is suspect. No reasoned, rational position needs good, loving parents to be kept completely in the dark, lest they argue against it.