Dr Cass might be right that there may be some children who are not going to change their minds no matter what.
But if that is true, there is still no way at all to identify who those children are. It doesn't matter what an 11 year old says or does, there is still a very high chance they will feel differently a decade or two later, if not less.
And even if that was true, even if there was a way to know that a child wouldn't change their mind later or would still be dysphoric years later, you would still have to weigh that up against the physical, medical and cognitive downsides of transition. This is a medical treatment - be that puberty blockers, cross-sex hormones or surgery - which creates an adult who is less healthy than they would have been if you had not intervened. I cannot see what benefit there is to any child that outweighs those downsides over the years. It's not normal ethical practice to give children drugs for their mental health that we have reason to believe will damage their physical health or fertility or cognition in later life.
At the very least the data linkage study needs to be completed and show that the long-term harms are not that serious and that there are some real benefits. Though I worry that it will turn out that so many young people have been lost to follow-up, and so much information lost to poor record keeping, that it may never be possible to draw any conclusions from it.