I suspect MS threatened them with a SmartScreen blackhole for the domain, I'm not surprised they pulled it.
Otherwise, that's the best we have.
In terms of a possible explanation for why GitLab would take an action, was it considered whether the (disturbed?) user violated GitLab's Terms of Service? Is the assumption that GitLab didn't just enforce their ToS, but that they're instead more likely to be secretly acquiescing to backroom bullying between companies over specific users?