I saw your screenshot but the context is very vague, and the post you shared the direct link to is not publicly accessible. So if there's further context, I cannot readily see it.
KBExit, I didn't realise that the cited thread was made private after I posted it. Strange; it's only a bug report. Apologies.
So are you asking why the Flarum support forums does not utilize it? Or are you asking why it doesn't come preshipped with Flarum software in general?
I've assumed that the GitLab OAuth integration is a first-party option, like the GitHub one is. If so, why is it not enabled on this instance? I'm asking because the cited (private) thread is about it not functioning:

Therefore, I want to know whether it's disabled here because of a bug, or whether that's coincidental.