It effectively means they see some guy (probably a minor too, doubly impressionable) who's maybe a little bit of a wuss, maybe soft, a bit emotionally sensitive, maybe likes some sort of slightly feminine or girly thing (even as just a phase) and this CONFIRMS they are a transwoman that hasn't fully come to terms with the TRUTH. It can extremely easily be used to manipulate impressionable and vulnerable people. It gives them some sort of answer to their problems and insecurities by just saying "You're actually trans, see? It all makes sense now. Now follow this step by step bible guide to becoming the woman you are truly meant to be, contact me or my friends so we can help you.". And then the cycle continues.
The very idea that a feminine guy or a tomboy can exist AND be perfectly fine with their assigned gender is somehow an impossible conclusion, you're just lumped in with the peanut gallery. You're either a jock, a proper woman, or a troon no exceptions or in-betweens exist. Which is absolutely infuriating hilarious when in the same breath we're supposed to just assume their are 17 million genders, "Hey dude" is misgendering, and xir/xhe are real pronouns by the same peanut gallery.
tl;dr: It is troon speak for "troon-in-progress"