Of course not. There is an assessment period. They are assessed for gender dysphoria, and if diagnosed they may be prescribed puberty blockers to treat their gender dysphoria. And what is the criteria for diagnosing a patient with gender dysphoria? Whether or not you think the criteria in the DSM are effective, at the end of the day these are the criteria that would be used to approve puberty blockers. If you think these criteria erroneously include kids who aren't "trans kids" then we're approving puberty blockers for patients who aren't "trans kids".
Of course a longer follow up time is strictly better, but it's not valid to simply fill in a gap in data with whatever better suits your worldview. If a study measuring rates of detransition follows up with patients for 10 years, then patients that detransition after 10 years would not be counted. Is it valid to point to a couple anecdotes of detransition, and then claim that the study's finding are false because there's loads of people who detransitioned after 10 years? Of course not. But that's the same flawed criticism you're making here.