It's not an ad hominem at all. It's not used as an argument against his statements about software. It was used as an example of his personality. How is a known

It's not an ad hominem at all. It's not used as an argument against his statements about software. It was used as an example of his personality. How is a known fact about his personality not a valid example of his personality? The statement being made was that the personality of rms is well known, so to act all shocked and surprised when he exhibits that personality is ludicrous. You could even say that it was the opposite of an ad hominem: an appeal to look beyond the known idiosyncrasies of Stallman and focus on, well, his statements about software. That's not an ad hominem.