There must be an underlying deterministic system. - We don’t know what it is yet. - Quantum mechanics is incomplete, not wrong — it hides deeper rules. - His belief is based on logic, not current experimental proof.
In short, he says we don’t know what it is, but it’s something out there.
I ain't no physicist but I learned about Bell's Theorem from a video where Feynman is explaining it in terms of boxes with buttons and lights on them, while dressed in a tracksuit. The audience keep asking questions so he goes over the idea about two dozen times, but that's not necessarily a bad thing in this case.
Strictly speaking, it can't be the result of local hidden variables under the assumption that you can make decisions in the detector that are independent of the experiment that you're doing. Usually those who argue for something like hidden variables try to find wriggle room in the latter part (that these decisions are somehow inherently correlated with what's being measured). But this is also deeply weird, because you could base these decisions in principle on information that has not had a causal relationship with the experiment since the start of the universe. (this idea is called superdeterminism)
AFAIK, at least one assumption needs to go.
There is a nice paper by N. David Mermin illustrating the incompatibility [2].
[1] https://en.m.wikipedia.org/wiki/Kochen%E2%80%93Specker_theor...
[2] https://www.physics.wisc.edu/courses/home/spring2020/407/exp...