The same code can have multiple overlapping sets of maintainers. For example, one team can be responsible for business logic while another team can manage core abstractions shared by many product teams. Yet another team may be responsible for upgrading to newer toolchains and language features. They'll all want to touch the same code but make different, roughly orthogonal changes to it.

Semver provides just a few bits of information, not nearly enough to cover the whole gamut of shared and distributed responsibility.

The comparison with multithreading is not really valid, since monorepos typically linearize history.