Using extreme ethical examples to justify a statement is how Godwin’s Law came about, way back in the 90s. It’s a well known tactic to make one’s own position sound reasonable, by implying that anyone is still arguing with you may as well be saying the Nazis weren’t so bad.
It’s a good rule of thumb that rather than making a logical argument, you are in fact bringing a rational discussion to an end by invoking slavery, Hitler, or other incontrovertible evil. They are not in any meaningful sense useful analogies for most discussions.
The guy wasn’t bringing up anything like slavery. Therefore Stallman’s response is quite likely unexpected to most people.