I'm not sure and I don't really care. But I'm assuming its just the admin's personal dislike of lemmy.ml. Regardless, I chose this instance because of it. I'm sure there are still small number of users acting in good faith on lemmy.ml, but I just don't wanna deal with their admins' tankie influence and the higher ratio of trolls on that instance.
It's unfortunate IMO. I thought it was just some kind of weird mbin problems causing lemmy.ml not to federate, then I mostly forgot about it. Although that server is not without problems I'm sure, it's a huge piece of the lemmy world to be missing and it's a pretty small world to begin with.