I'd rather call it in-house than proprietary. Maintaining or even publishing a project in the open is a lot of work, and it's not like there aren't a lot of open-source forums available, so I can imagine the maintainers deciding it's really not worth it.
In fact, the comment system of LWN is in-house as well, and I doubt anyone would accuse them of proprietary sympathies. They're getting there, but it's just nor a priority, or even that important in the open source world.
I'd rather call it in-house than proprietary. Maintaining or even publishing a project in the open is a lot of work, and it's not like there aren't a lot of open-source forums available, so I can imagine the maintainers deciding it's really not worth it.
In fact, the comment system of LWN is in-house as well, and I doubt anyone would accuse them of proprietary sympathies. They're getting there, but it's just nor a priority, or even that important in the open source world.