> These always ignore the fact that a large chunk (maybe the majority?) of OSS development happens during working hours at an existing employer.

Do you have any sources for this? It doesn’t seem right to make a vague statement (a large chunk) which sounds bad, nor a somewhat less vague conjecture (maybe the majority?) without sources.

In my very limited experience, any developer who is significantly supporting OSS with code contributions is a high contributor for their employer, and if they don’t always do the agreed-upon number of hours/week for their employer, I doubt it’s far off. So I disagree with you anecdotally, which doesn’t matter much, and I question your sources, which matters more.