Fundamentalist lefties aren't uncommon, but Stallman isn't really typical for that group, as he's a fundamentalist of a particularly niche ideology that, while on the left in a broad sense, isn't typical of leftism (fundamentalist or otherwise.)
I saw an old interview where RMS goes bananas when the interviewer calls him a communist.
[edit] if you follow any of RMS's thinking he is generally quite precise and careful in his writing, choosing words carefully, eschewing ready made labels and coining his own with more precise definitions.
So it would not surprise that referring to Stallman as say a "lefty", "communist", "libertarian" or a whatever would annoy him, since there is no good consensus on what those terms actually mean.
I would posit that the use of over-broad popularly contentious labels is generally counter-productive to actual thinking and one of the major problems in modern political discourse - it promotes tribalism over thinking about issues.
Labelling is a poor substitute for reasoning.
So yeah, he also defined his version of "freedom", but labelling sounds very RMS to me. Also the strict wording of "ethical" software, meaning only the licence he favors is ethical and good and everything else, not.
RMS has his faults but ambiguity is not one of them. He really bends over backwards to be careful about explaining what he means.
Although I'm sympathetic to that idea myself, that just isn't true in Stallman's case. He's a Bernie/Stein voter and he's read something by Emma Goldman, but I think that's the extent of his "lefty" gland.
An actual libertarian would be pro: "there's no copyright nor copyleft; my computer -> my rules, I do whatever I want with whatever files I empirically have. And the only way to decisively stop me from doing that is to actually kill me."
I am saddened by this occurrence, and am worried about the ability to pressure people like Stallman like this.
If a critical mass of people refuse to work with you, that is itself a mishandling of the project. Getting people to work with you is one of the most important responsibilities of a leader.
Also - you're right that this is an important characteristic, but if there is active outside pressure to get people to _not_ work with you, that should not be ignored when evaluating someone's ability to lead.