And on the other side: are there major employers who implement rigorous coding interviews who... don't have a history of producing good software? I really can't think of any.
That's my point: the evidence is really stacked in favor of the FAANG scheme. So pointing to a handful of small outfits doesn't really sway my impression; those are just expected outliers. I've worked at smaller companies with terrible hiring success, FWIW; they ended up successful because of a few lucky hires.