I don't think you are using that right. Saying someone is bringing a bad vibe to your project, as the point he has a bad vibe is just stating the conclusion.

Like if someone calls you a bad programmer and doesn't hire you as a programmer, isn't ad hominem

It isn't. The grapevine stuff was unnecessary and should have been cut out but there is plenty of concrete stuff in there as well.

Part of it might feel like ad hominem, but I think the bluntness is justified in this case. It provides some pretty important context on the situation.

As an outside observer who knew little about Zig and Bun, the impression I got is that the rewrite came out of nowhere, and that the Zig community was surprised by it. The post makes it quite clear that this wasn't the case, and describes how the actions and overall mentality of Bun's creator already suggested such a thing was likely to happen, straight from the mouth of someone who worked with him and isn't afraid to voice his thoughts. Not everything needs to be PR speak.