Terfs do the they/them misgenderinjg thing all the time when they want plausible deniability, it's incredibly naive and completely removed from reality to assume that transphobia, especially lowkey internalized transphobia that people are in denial about, always has to be blatantly obvious.

Regardless of that, the only reasonable and not inherently transphobic reaction to misgendering somebody is to politely correct yourself. Getting snarky or starting a debate about this where you argue why it was ok to misgender a trans person means you have issues with this that you need to work through.