What you say is true, but the open source model does make it easier for people to subscribe to a subset of threads, issues, projects, etc that they find relevan

What you say is true, but the open source model does make it easier for people to subscribe to a subset of threads, issues, projects, etc that they find relevant to their goals, and to follow and contribute to those selectively. In other words: core project team(s) may follow traditional organizational practices, but it's also possible for them to accept contributions from pro-bono specialist interests. (that's theoretically possible for companies using inner-source approaches too, although the audience size in those situations is likely to be smaller)