Well actually the jury is still out on that. The clinical data I've seen does suggest its about 50/50 - point is I would argue that if people are allowed to grow up the way they want to, and then decide to detransition of their own free will, they have suffered no ill effects from being allowed the freedom to explore.
Indeed they may go on to be better, human beings, more secure in their chosen identity, because of the insights they have gained... whereas if you prevent one of those of us who DIDNT change our minds THEN you DO ENORMOUS harm to us.
So the right argument is not to try and falsify the data to show something that I honestly dont think it does, its to point out that one way the only effect is that some people, who later go on to revert of their own free will, have additional experience in life, where the others are protected from suffering an enormous amount of harm.
So the way to defeat this is to argue that allowing freedom does no one any harm - and may do a few a lot of good, whereas denying help because you cant be sure if this is "just a phase" or not - WILL cause harm.
I'm sure back in 1965 when my parents allowed me to express my female nature they were 100% sure I would revert... and for a short time in my early adult years, I did - but it was still the right thing to let me call the shots at every turn and i'm very grateful they did.