I disagree that the comment is harmful or transphobic. It isn’t punching down or attacking trans people, it’s making fun of the insane belief that school teachers are performing medical procedures on children without their parents’ knowledge or consent.

I want to make a few things clear before I go any further: body / gender dysmorphia is not something that magically begins at the age of 18, and the idea that treatment for it should be held off until that age is as ridiculous as implying depression or anxiety not be treated until then. There’s also a myriad of reasons as to why someone under 18 would want to seek this treatment without their parents’ knowledge.

That being said, the extent to which treatment and discretion should be provided is obviously a hot topic in today’s political climate, and whether you’re in the group that supports or condemns it, nowhere are there schools with teachers performing secret transition surgeries on children without their parents’ knowledge. The poster is not making fun of an actual thing that is happening or even has a high likelihood of happening; they’re making fun of the lunacy and (often intentional) failure to understand the actual argument, and highlighting the strawman behind their defense.

I was going to liken the comment to someone making fun of an anti-Semitic news article by saying something like, “my Jewish friends always say they’re too busy running the world to hang out with me”, which could be taken at face value as an anti-Semitic joke; but the comment in your OP doesn’t even say anything negative about trans people (or those that provide treatment for them, for that matter). The only ones really being mocked are the people that believe the second statement is true.