to say they are 'fully reversible' is plainly not true
It is true, but only in a very specific technical sense about the treatment.
The treatment is a "puberty blocker". When you stop taking it, it stops blocking puberty.
That is "reversible".
Doesn't necessarily mean you will go through normal puberty when you stop taking it though. You likely would if you stopped early enough - if it was precocious puberty treatment - but if you stopped at 17/18?
So, it's technically true, but totally misleading to a layperson, like parents.
Looking at wiktionary, for reversible, we have:
1. Able to be reversed.
2. (of clothing) Able to be worn inside out.
3. (chemistry, of a chemical reaction) Capable of proceeding in either direction.
4. (physics, of a phase change) Capable of returning to the original state.
5. (thermodynamics) Capable of returning to the original state without consumption of free energy and increase of entropy.
It's not shown there, but what we're talking about is a medical sense akin to entries 3-5.
But it's not the common usage 1 - "we can undo what we did and carry on as if it never happened".