Sure, but it's still a counterexample for "you can't define it because it means sanitizers can't warn for it". Sanitizers can warn for it; you "just" get a worse signal-to-noise ratio.
> It'd be less "false positive rate too high", more "it disallows you to use a genuine language feature that is actually useful"
I'm not sure I see the distinction? Flagging a correct use of a language feature as incorrect is more or less the definition of a false positive, so if intentional signed overflows get a reasonable amount of use then that'd presumably result in an unacceptably noisy check to be enabled by default.
> defeating the point of defining signed overflow in the first place.
As for unsigned overflow checks I'd imagine the intent is that one would enable that particular check if you think that the corresponding overflow is more likely to be unintentional than not, and in the cases where it actually is intentional you can suppress the check.