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.