They will often say things like "yes, we understand you are attracted to dick/pussy, but what if the person is fully 'transitioned'?"

I don't think it's just truscum people saying this. I think there's plenty of AGPs and transtrenders who say things like this because they're more interested in looking for gotchas than in building a coherent movement. They want to make anyone who asks any questions or asserts any boundaries look like a bigot. To do so, they're relying on the general public not knowing about the reality of transition particularly with regard to genital surgery, but also cross-sex hormones, puberty blockers, and double mastectomies. If the general public saw what went into surgery resulting in "neovaginas" and "neophalluses" as well as what the results look like, they wouldn't buy the argument that "there's no difference." I think a lot of people who push these narratives are really young and have no sexual history to speak of so it's easy for them to say "there's no difference" or that "people can't tell." In Irreversible Damage, Abigail Shrier covered that the vast majority of teenage girls who transition to trans men have no sexual history and haven't even kissed anyone.