Honestly, I would argue that git submodule should count as a package manager. I simply list out the repos I want to pull in and update them as needed.

I see your point, but I think that when developers refer to package managers, implicitly that means accessing standalone precompiled binaries that are ready to just be integrated into a project.

With git submodules, unless they are used to track standalone projects or even precompiled binaries, you still have to resolve their dependencies, which is the responsibility of a package manager and the main reason they are used.