> I sincerely have no idea why people believe so strongly that a human-level AI can build a superhuman AI. ...how on earth would such an AI even know it succeeded?
This touches on one of the few good reasons to be less ardent about AI/AGI: "intelligence" is not very well-defined and we don't have very good ways of measuring it. I don't think this is a total blocker, but it might present difficulties. What if our current approach ends up creating super-autism instead of super-intelligence? There's a long history (starting with Asimov) of drilling down into how vague things get when you start trying to draw clean lines around AI and its implications and those questions are yet to be definitively answered.
However, your broader point seems to imply that you can't "bootstrap" intelligence, which I don't find convincing. Humans, after all, could barely master fire a few hundred thousand years ago, and now we have an understanding of the universe that the earliest humans were incapable of comprehending on even a basic level. It's obvious to me that simpler things are capable of building more complex things; blind evolution can do it, so there's nothing in physics preventing intelligence bootstrapping. We also have the ability to use intellectual division of labor to build tools that vastly enhance our abilities as a species. The human brain as hardware is far from some impassable apex; hardware can always be used to build better hardware, much like the earliest CPUs were themselves used to design better CPUs.