Just because the author can't think of a way to cause mass death with an engineered virus, doesn't mean an AI won't be able to think of one. (There are several biological threats the author hasn't considered. I'll just leave it at that.)
It's the fact that the complexity of molecular physics scales exponentially in particle number. No amount of thinking is going to punch through that. You've got to do experiments. The slow loop through reality is not optional, and in the case of "superviruses" would require a lot of iterative experimental development in humans.
(And please knock off the bio infohazard act - I routinely get asked about scenarios that AIxBio safety people cook up and they always involve some howler misunderstanding of basic facts in biology or medicine. Just talk about an idea if you have it.)