The entire thing is beyond confusing. So Anthropic refers to an open-sourced project and credit it in the original article.

I don't know how the author is pissed off. Only thing the author aware is Claude agents researched Mononoke solution, then it claims a closed source repo is a plagiarism case.

I don't get it, and I'm seeing more and more cope here.

My read of the author's position is that the people who built Mononoke at Meta did all the hard engineering work that made Scion possible, while the work that happened at Anthropic was mere code-monkeying unworthy of note. This is decidedly not the sense you would get from reading Anthropic's blog post, so they feel that that post misrepresented what happened. I am skeptical that this is really an accurate characterization, but I can see how a person might get there, especially if they were involved in the work at Meta and so have a natural bias towards weighting its significance more heavily.