The problem with naive self-identification as the go-to for telling who's trans and who isn't is that eggs exist.
People who have repressed their trans identity so hard that they will insist that they're cis even in the face of overwhelming signs to the contrary ("I'm not really trans just because I cry myself to sleep wishing I'd been born different") are a real group that is actively subject to marginalization for being trans.
Trans liberation politics can't be formulated in a way that leaves these people out just to respect their incorrect self-identification.
There just isn't an equivalence between doubting the stated identity of someone who says they're trans and someone who says they're cis.
In the first case, you're enforcing cisnormativity. In the second case, you're either making an incorrect guess with no real consequences, or you're providing a trans person a shot at killing the cop that society put in their head.