> Put in another way, if someone wanted the ability to touch every line of code (in the upstream sense), they would have to be a member of some non trivial amount of communities.
On a typical distro you can just download sources and start hacking, you don't need to be member of any community.
While something like Debian might not be monorepo in the strictest sense, on a conceptual level it is very close. They still have all the sources under their control and are not dependent on anything outside. They are at full liberty to accept or reject any patches regardless of where they come from, from "upstream" or "downstream".
This idea that distros are actually independent full-featured operating systems is an idea that I think is getting forgotten way too often. Distros are (or rather can be) much more than mere repackaging of upstream software.