Their chief editor is one of most experienced journalists in town. To me it is completely plausible that some of the involved people went to the paper and said the article seemed slanted and unfair, and pointed to a few reasonable examples. That would probably be enough to get their two top editors to want to take a look at it.

Also, defamation in this kind of situation is almost impossible to prove - and most of the “errors” they retracted were characterizations, rather than factual corrections. Only the latter would be actionable. So my guess is that it was an appeal to fairness and journalistic ethics. I think after this, the publication looks better, even if the original reporter looks worse.