Correct it is not open source, but it hosted by Chaos, AFAIK. It’s canned software a company can buy (or rent) and run. Only the hoster has the data.
Discourse is the same way. Discord is completely different, and is for chat and collaboration, similar to Slack, but for gamers (in general, though they are trying to push toward other uses).
I cannot stand the Discourse UI. It seems so clunky, trying to be a phone app. Too much rollover crap, and other completely unnecessarily “modern” web garbage. I would much have this or PHPBB, or any other traditional forum that doesn’t try to act like a phone app for teenagers. It makes this narrow centered layout, which looks absurd on a large screen-- looks like an iPhone emulator. That is probably my least favorite forum. Thinkbox has used it for a while, and it is a chore to work with.
But everyone has their own opinion 8)
A company should NEVER build a system themselves, if good alternatives with little downsides exist. It would be a waste of money and resources to hire a developer and take a away resources from UX, design and others to create something that is readily available.
Sorry, not saying they should write their own forum software, but running it is not so hard.
So I am not sure what your point is, even though I agree whatever forum provider/software they use it probably shouldn’t be anything by Google or Meta. They don’t offer any forum software I am aware of. I guess you might mean a Facebook User Group.
Google Groups (did they kill that?), or Groups.io, or any FB anything. Facebook is the most egregious, since you can’t use it at all without having a FB account. While, obviously not a forum, a lot of companies use FB, which drives me nuts.
The point? Keep it as an independent forum without some third party data broker running it, and fix whatever the occasional error is, as this works well here.