The point is that GPC doesn't want to use something proprietary.

Lol.

Ok, but they’re complaining about an open-source project; they can fork zotero and make it point to their own sync server.

"Just fork it" is dismissive and misses the point. The criticism remains valid even if the maintainers refuse to act on it for whatever reason. Even if I were to invest the time to fork the project, the criticism would _still_ remain valid as long as upstream didn't take steps to fix the problem.