Fabrice Bellard is not a 10x engineer, he is a 100x engineer. You could attach him to a good people manager and either build a team around him or allow him to work independently on a project that he finds exciting that also aligns with company goals.
Bellard wouldn't apply and be interviewed like some Stanford grad. He would be head hunted and told he can do whatever he wants and receive a massive amount of compensation.
I'm not sure why you woulf assert he wouldn't pass the interview that seems totally outrageous.
Given his alma mater and the way the French education system works, he performed too-of-France at “solve math problems on a blackboard in front of someone” after two years of grinding math problems including extensive practice for the aforementioned “solve math problems on a blackboard in front of someone”. I think he could manage. FAANG interview is basically a CS khôlle.
Put Terry Davis (again him) as senior manager at Apple, and see the result.
From my point of view, Terry has the same level and approaches as Fabrice.
It does not guarantee at all that he is going to be more productive than 100 engineers as you directly claim.
It makes them good in what they like to do (writing obfuscated or low-level code, or implementing from scratch from specifications) as art or creativity.
I am definitely not talking about art.
When I refer to 100x engineer, I'm referring to the impact that QEMU and FFmpeg have had on the world. I would be surprised if anyone who is familiar with these two projects would disagree that they have been highly impactful.
EDIT: Fair enough, I think he would be very productive due to useful contributions, at the end I agree with you.