[brechtm]

I don't quite understand this one. "Now you’ll have to create a new commit...". At this point, you haven't yet created a top-level project commit at all.

That sentence intends to illustrate what can happen when you forget to push your submodule commit before commiting on the top-level repository.

Why was submodule better than just using a single repo and multiple remotes in this case?

I'm not sure what you mean with this.

Or put another way, what did you gain by having separate repos if all the repos branch and come back in exactly the same way.

We used submodules because the code in the submodules was shared with another team.

You want to also branch the submodules so as not to break another branch.