I had read the discussion in the past and saw they already had GPL related dependencies they had pulled into the repository. I don't know the full history or the contributors project goals. I usually don't find GPL code hard to avoid in normal MIT projects already separated from the GPL, but I can't state an opinion because I haven't tried to understand the background. This is to say, I don't find it completely convincing, and view it partially related to the contributors just liking the GPL.

I wouldn't personally make this choice because the project competes with an MIT licensed version. As an example, one of their goals is federation which might bring companies which realize they don't need GitHub to promote their projects or collaborate. If their competition does the same, those companies might be more likely to choose the permissively licensed version instead. Though, this is a hypothetical example, and I think I'm just negatively biased against the GPL.