I think the fact that it's not unilaterally lethal, but typically so, is the reason for the quotes. If it were literally lethal, no quotes, it would imply that

I think the fact that it's not unilaterally lethal, but typically so, is the reason for the quotes. If it were literally lethal, no quotes, it would imply that any human that drank that quantity would die and that the subject here is simply superhuman. It is totally offtopic, but I consider their grammar absolutely correct.