Assuming AI stays on the trajectory they think it will doesn't mean they assume it will be infinite exponential growth. If you know for a fact it's sigmoid curve, presumably the path you think it will continue on is the sigmoid curve. The trillion dollar question is does performance plateau before or after AI can do the really exciting stuff, and while I may not agree with it myself the more optimistic position is not an unreasonable belief.

Also you can most certainly fit a sigmoid function only from past data points. Any projection will obviously have error, but your error at any given point should be smaller than for an exponential function with the same sampling.