It's complete and utter horseshit. The Cass review claimed that there was insufficient evidence re: long term impact of puberty blockers. That's because Hilary Cass excluded all the evidence based on the fact that none of the studies had a placebo group, despite the fact that this would be literally impossible to achieve and it not a requirement of good science. It is a tested usage with multiple international studies over a period of decades, Cass just threw them all out because she's a hideous bigot who wanted to engineer a specific result.
His claim that the medication is safe for most uses except this one defies basic logic and is just a repugnant prick who accidentally joined the wrong party couching his prejudice in terms that appeal to Telegraph readers. Essentially, he's saying that we need a certain number of people, given the medication in youth to die of old age and nothing else before he'll sign it off - so 80 years plus worth of data. Unscientific bullshit.
He's a disgusting bloodthirsty oaf, using the political concept of "The Child" to gain consent for his bigotry. For reference - see how much he cares about Palestinian children compared to Ukrainian children.
Cass was the only person asked to lead the report - the bigoted political class went straight for the person who would give them the result they wanted. They only care about keeping the press onside. Trans people are basically the acceptable victims of Labour's win and if you voted this, you are responsible for it.