Well, with Subversion you can sort of have different repositories in one repository... I think just about nobody had a checkout of the monorepository (it was too large), so changes (e.g. in public API) in several parts of the stack were not perfectly synchronized. There was one check-in per atomic repository for such things.

Greg KH gives a pretty good talk about the rate of kernel development fairly regularly[1]. But yes, it's been consistently getting faster every kernel release.

[1]: https://www.youtube.com/watch?v=vyenmLqJQjs

Absolutely. It's a crude estimation. If anything, it's going to be even more commits per release. The point being, it's impressive that those many changes go into a single release of something as critical as the Linux kernel.