This is why, this statement means that you don’t view trans people who haven’t “finished transitioning” as their actual gender. This is a transphobic and pretty reductive understanding of how transition works (albeit one that some trans people hold themselves, usually transmeds). I won’t write an entire essay on why but here’s just a small bit to chew on: consider that a “finished transition” is very different from person to person and some people might never consider it finished. Some people only want to socially transition, others have to medicate for their entire lives, both could consider reaching their ideal state “finished” or they could consider it a continual work in progress.
Also,
This isn’t defense, this behavior actively makes things worse for trans people.
The idea that pointing out someone’s transphobia will somehow support the transphobes is laughable. If being called out is seriously enough to make someone stop supporting trans people then their support was conditional and only surface-level.
Staying civil, as you suggest, is what actually helps conservatives since it allows their views to go unchallenged when their views are bad and should be challenged. Part of this includes challenging people who may think they are supportive but harbour transphobic beliefs that they haven’t analysed fully. And these wrong beliefs can have actual harm. As a simple example, there are a lot of “allies” who say that trans people are their gender but not their sex, which is a belief that can harm trans people when brought into a medical context where our bodies are (if on HRT) closer to those of our actual gender than to our AGAB.