I have no strong opinion on whether a "hard takeoff" is probable. (Because I haven't thought about it a lot, not because I think the evidence is exquisitely balanced.) I don't see any particular reason to think that protein folding is the only possible route to a "hard takeoff".

What is alleged to make for an intelligence explosion is having a somewhat-superhuman AI that's able to modify itself or make new AIs reasonably quickly. A solution to the protein folding problem might offer one way to make new AIs much more capable than oneself, I suppose, but it's hardly the only way one can envisage.