If you are confused as to how the statement "trans women are trans women" could be transphobic, it might help to imagine the question was "do you think black people are people" and the reply was "black people are black people." It's a true statement, and also one that many would call racist as a response to that question. You may not agree that these are equivalent, but that is the position you are arguing against, not morons who don't understand that there are differences.