I think Stallman started on the wrong foot. He used his own definition of "sexual assault" - one that requires physical violence - when "sexual assault" generally means something more like "sex with someone without their consent." He criticized people for using "assault" for something where there was no described physical violence, and asserted that "sexual assault" was a meaningless term.
For those who understand "sexual assault" to mean "lack of consent", rejecting the first term is the same as rejecting the the concept of "consent" in a sexual relationship.
He could have criticized the term, like how he criticizes the term "intellectual property", but instead he rejected the term as being meaningful for this situation. And I want to stress, he used his own (folk) definition for 'sexual assault' in order to disparage how others were (correctly) using the term. How can he argue for a clear description when he doesn't even know that people are using a clear description?
His second wrong foot was in arguing that since age-of-consent laws vary, so sex between a 73-year-old and a 17-year-old is legal, shouldn't be counted as completely immoral. However, the same could apply to Epstein's pedophilia, so this is an indirect support of pedophilia.
Further, this was coerced sex with a minor, so age-of-consent laws don't apply.
This was all part of a defense of Minsky, who (Stallman argues) wasn't violent and likely assumed she was willing. That's the third wrong foot - "I thought she was 18" isn't a legal defense for age-of-consent laws. But really, it's several feet along the wrong path, as the question is not about age-of-consent - which may have been legal at the time Minksy visited the VI - but about sex with a sex slave, or commercial sex with a 17-year-old.
Note that under current VI law, sex between a 70+-year-old and a 17-year-old is classified as second-degree rape, so while it's true that not everything is child rape, in this case the distinction is very narrow. And Stallman was not making narrow arguments about VI law at the time the sex occurred.
Stallman presented what he thought was the most likely scenario, but didn't consider other scenarios. He assumed that Minsky did have sex with a 17-year-old.
In his hypothetical scenario, did Stallman think that Minsky did not consider this was a set-up for blackmail? How many other teens has Minsky had sex with? Why couldn't it be that Minsky knew that Epstein set him up with a prostitute, under a plausible deniability arrangement? (Statutory rape laws are set up to reject a plausible deniability argument.) These seem quite reasonable questions, but unaddressed.
To get to your comments, the issue is that there are clear descriptions - well, as clear as anything involving human nature - codified in law and used in wider discussions on ethics and morality. Stallman doesn't appear to understand these, and doesn't seem to believe that others do, so he rejects the basis for complaining about sexual assault at all.
He then talked about it at the wrong place, and the wrong time. What's the worst that could have happened had he written nothing? Very little. It would be a footnote in Minsky biographies. But at a time when more and more unethical behavior at MIT comes to light, well, it's not good timing. Even worse was his own long history of poor social interactions with women, which Minsky and the AI lab had helped protect, as well as his standing as the guiding light behind the free software movement.
With Minsky gone, and the spotlight on the MIT senior staff for their actions, he no longer had the buffer when he caused that bright light to shine on himself. I think this can be thought of as a change in the social landscape causing a "cusp catastrophe".