if he actually was, then it's not a personal attack but stating facts about a person - there's a meaningful difference.

A personal attack has no informational value (about the accused), while bringing to light bad qualities of a person does - especially if those qualities are related to the issue at hand, which in this particular situation seems to be the case.

Again, the distinction between a personal attack and proper characterisation of bad qualities rests on actual facts i.e. was it slander or was he really a bad manager?

I don't know but I personally give less probability to the former than the latter.

Managing a very public project that advertises Zig and doing so poorly is a matter of fact, is it not?

That was what ZSF try to implies. And would have been much better if it was written as such. But the way it is currently framed is a personal attack.