Lurking American academic here; I teach at a very similar (and similarly kind of mediocre) college, but in a different field. I've read and written a lot of the kind of evaluation documents that got excerpted; they're generally confidential but the person at the heart of the case does get copies. Most of us don't then go post bits on Twitter, but...
One of the central questions in a tenure case is whether there is a reasonable expectation for future scholarly "productivity." Which is generally measured in terms of academic publications. It's expected that the department will report on both strengths and weaknesses; seems to me like they were pretty honest about the directions RM's intellectual work was heading, and that standard academic philosophy was less of a goal than it might be, which would be a legitimate worry from the point of view of the institution.
RM has tenure; yes, that means a job for life, or at least until really serious professional misconduct, or the institution eliminates your entire department, or itself closes down. My (rapid and superficial) read of the tweets was that they were a humble brag: look at how my benighted department tried put me down, in this document that I am totally inappropriately posting here, but NOW I'm going to be a star! And I'm even better at philosophizing than I used to be! That'll show them!
Post-tenure evaluation has become a thing, but not much of one, because tenure does mean, well, a job for life. (My department characterized my last set of documents as "a cry for help." O rilly? This year's sabbatical in the UK has done me a world of good, actually...)