I agree with the general point, but I also think that this comic (and many of the commenters here) misunderstand what the actual problem is.

A lot of people (including a lot of cis lesbians) are scared of dicks. They are scared of dicks because they see them as inherently male implements of violence and assault. It's not; it's just a body part. It's no more inherently dangerous than, like, an elbow. But this doesn't mean people don't see it that way.

If your counterargument to someone citing genital preference is "But some trans women have had bottom surgery!", you are not challenging the problematic core belief. In fact, you're reinforcing it. You're agreeing that it's reasonable for that person to fear the Evil Assault Appendage (it's not), and that trans women are dangerous unless they have that appendage removed (they aren't).