Having had used both, Google's implementation is IMO the superior version of monorepo. Really, Google's Engineering Systems are just better than anything that I have ever used anywhere else.
If you want a centralized, trunk-based version control, don't use git.
It's funny how each company decides to solve these problems.
Google called in the computer scientists and designed a better centralized vcs for their purposes. Good on them. It'd be great if they open sourced it. So typical of Google to invent their own thing and keep it private.
Microsoft took the most popular vcs (git), and inserted a shim layer to make it compatible with their use case. How expected that Microsoft would build a compatibility shim that attempts to hide complexity from the end user.
Meanwhile, Linux and Git are plugging along just fine, in their own separate repos, even though many people work on both projects.
It would be hard to open source if its dependencies only exist on Borg. I think that was a problem with Blaze (now Bazel).
Yeah like their build system... Bazel, that's completely closed source.