Lost me at "And lots of people learned that monorepos are really painful, because past a certain size they just stop scaling." Plenty of counterexamples of monorepo projects much larger than Linux kernel.
If you're arguing it's not a (implied-to-be-evil) monorepo because, well, it's really managed as independent fiefs who do all their work in separate integration stages... well, that's basically how every other major monorepo project does it. So if Linux isn't a monorepo, then there's no such thing as a monorepo in practice.