That's the problem with the internet today. It's too toxic to hold any meaningful public discussion.
I'd bet that RMS was open to discussing the point he expressed. Because that's what you do on an old-fashioned mailing list. But apparently he got no response to his email. Instead Selam jumped the gun and started, essentially, a shaming campaign on medium.
This kind of attitude essentially hushes people who can actually hold a meaningful online discussion, and perhaps revising their starting opinions after one. Instead we only get to hear the voices of people that are righteous about their opinions—be right or wrong.
Internet has transformed from an open forum to a shouting contest; if you can't join a big enough shouting group, no need to waste your energy participating.
He was defending a 70 year old who had sex with a coerced child by trying to say that the word "assault" shouldn't carry either its everyday English meaning or its more specific legal meaning, but should instead have some other meaning that he created.
It's also funny that you complain about RMS twisting the "everyday English" meaning of "assault" while at the same time you call "child" someone who you'd most definitely call a "teenager" or "youngster" if you met in person.
This urge to abuse words in order to sensationalize an already perfectly valid argument is what RMS was pointing out in the first place.