Earlier quoted context omitted.
Sidetrack, but I'm curious. What do you mean by "a postulated mechanism by which it could actually happen that isn't pure SF"? It reads as if you're asking for a mechanism for a future technological event explained purely in terms of present technology. The whole "existential risk" concerns aren't about what present systems can do, they're about what future self-improving system might do. If we postulate this hypothe…
Not OP, but I agree with the point you question, and my rationale for so doing is that I've yet to see a compelling argument that a self-improving system is other than the software version of a perpetual-motion machine. Those seemed plausible enough, too, when thermodynamics was as ill-understood as information dynamics is now.
Not saying the parallel actually carries any meaning, just pointing out that you can make multiple analogies to physics and they don't really tell you anything one way or the other.