It's interesting that people often become highly emotionally engaged, to the point of outrage, about transgender teenagers getting surgery, when that's not actually happening. And except in the case of older teens, hormone replacement therapy (HRT) isn't happening either. Instead, some teens want puberty blockers to push back the need for HRT a few years.
You'd think TERFs would be happy about that, because it gives kids breathing room and potentially even time to decide they were wrong about wanting to transition. After all, TERFs often claim trans kids don't know what they're doing or thinking.
But instead of embracing the sensibility and practicality of puberty blockers, TERFs make up stories about how they might not be safe, despite a decades-long track record of safety.
Your hypothesis for why makes quite a lot of sense.