The joke is confusing if you're not well-acquainted with US abortion politics, and, if you read the entire email thread, there were actually quite a few people who didn't get the joke properly (you can include myself in that list as another example).
RMS's stance is quite galling here. First, he's making it very adamant that the entire purpose of the joke is political commentary. Second, he's resisting any mechanism to actually make it clear that it is political commentary. Finally, he's effectively trying to use dictatorial powers to make his views go through over the strident objections of the consensus view of the maintainers.
Except, I don't think RMS intended this as a "joke". I honestly think he was dead serious, but worried that other people would interpret it as a joke and thus be inclined to remove it, hence his "do not remove" comment.
Technical documentation and reference is really not the place for political commentary. It adds nothing and is distracting. Technical documentation should be just that: documentation; not political diatribe. The politics would be better left to a blog post or something of the sorts.
If he was dead serious, he's an idiot. There was never any proposal to ban all usage of the word 'abort' in the sense of 'bring to a premature end because of a problem or fault' or as a noun meaning 'an act of aborting a flight, space mission, or other enterprise'.
RMS's stance is quite galling here. First, he's making it very adamant that the entire purpose of the joke is political commentary. Second, he's resisting any mechanism to actually make it clear that it is political commentary. Finally, he's effectively trying to use dictatorial powers to make his views go through over the strident objections of the consensus view of the maintainers.