Yes exponential is only an approximation of the first part of S curves. And this author claims that he understands the exponential better than others…
if the money dries up because the investors lose faith on the exponential continuing, then his future looks much dimmer
The disease is obviously also limited by the total amount of hosts, but I assume there's also the "bottom" limit - i.e. the resource consumption of already-infected hosts.
Back on the subject of AI, I think the flat part of the curve has always been in sight. Transformers can achieve human performance in some, even many respects, but they're like children who have to spend a million years in grade school to learn their multiplication tables. We will have to figure out why that is the case and how to improve upon it drastically before this stuff really starts to pay off. I'm sure we will but we'll be on a completely different S-shaped curve at that point.