I think it's monorepo vs multi-repo, and since Linux maintainers update the drivers (I believe) when internal APIs change, and the internal APIs are not stable, monorepo seems more practical.
"how the source code is stored" is what leads to "wow, the linux kernel is really big". Nobody's measuring kernel size by the number of resident pages it takes up.
The author is the same (Linus), so it makes sense that he would design a source control system that supported the monorepo he created over the prior 15-ish years.