> If a company turns away many developers who would be excellent hires but consistently meets its product/engineering goals and hires more good developers than bad developers overall, their recruiting process certainly has a positive signal. It's obviously inefficient, but it couldn't be meaningless.

Actually, that result would be precisely what you would expect from a process that fails to disprove the null hypothesis, and is therefore meaningless. You'd see them hiring essentially a random sample of engineers with the same distribution as the whole population.

I leave it to someone better at statistics than I am to figure out what an actually effective hiring process looks like, but I think the correlation between "is a relatable person" and hiring is much higher than "is a good engineer".