I'm pretty sure he said he's harmless because of how detached from reality he is. Think about it, if this autist doesn't get the tonal whiplash by comparing Maus to Zootopia, do you think he ever leaves his mother's house? He's probably completely isolated from the outside world, therefore he is not a threat because of how isolated he is.
That's how I understood that reasoning. He never leaves his mother's house, all that he yaps about is what he reads on the Internet. His only form of contact with the outside world is that computer. He has no personal life besides his room, his toys and his computer, therefore he cannot rape an animal because he'll never get a chance to due to his lifestyle.