So how they did in one job interview says more about their intelligence, to you, than GP's witness from knowing them for possibly years?

I think your weights are way off.

I think being able to pass a FAANG level coding interview with 3 months prep is a better indicator of their intelligence than one coworker's opinion, which could be clouded with biases.

And I think the vaunted "FAANG level coding interview" is a prime example of a setting where three months of hard swotting can make even a relatively dim bulb shine brightly.