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.
Actually, my experience is that monorepos scale better than non-monorepos. Probably the largest project that isn't a monorepo is OpenJDK--and even then, that's only barely true, as the entire JDK (effectively, rt.jar) is itself a single repository.
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.
