Considering that there is
another XKCD comic about furries, in which the creator seems to be informed about what a furry actually is, I don't think he has assumed anything. Rather, he is just making a joke out of both the sexual side of the fandom as well as those who troll it.
I understand what you're saying, and I see it can be confused as holding furries in a negative light, but sometimes it doesn't hurt to laugh at ourselves. Nobody can deny there is a sexual side to the fandom, but even if that isn't what "defines" a furry, it doesn't mean we can't have a sense of humor about it once in a while. The overall message of the comic is that even the most "devious" or "perverted" of furries aren't any more devious or perverted than any other devious or perverted non-furry, and that people should give them a break.
It's bad enough to waste time and energy getting upset over people who are actually intending to attack the reputation of furries, it's worse to waste time and energy getting upset at non-furries who mean well and try to put in a good word for us, even if they don't entirely "get it."