If the license allows you to freely read, modify, and distribute the source, it's open source. If we add "upstream welcomes your contributions" as another criterion, that would disqualify a lot of open source projects including all of the abandoned projects out there.

The Linux kernel is most certainly open source but their criteria to accept contributions is no more open than this or many other projects.

The maintainer(s) get(s) PR approval. Big features that change the direction of the tool are welcome in forks but require maintainers to agree if merged upstream.