It's difficult to prove that something that has never been done before is possible, until it has been done. I personally don't see any fundamental limitations that would limit non-biological intelligence to human-level intelligence, for your preferred definition of intelligence.
> And why we are we so sure GPT-x is the path there.
It may or may not be. Regardless, the capabilities of AIs (not just GPTs) are improving exponentially currently.
> To human level intelligence sure, but it's not obvious to me that it will enable superhuman AI
If you think GPTs can get to human-level intelligence, why would the improvement stop at that arbitrary point?