Using the word assault causes people to believe a different thing happened. It says nothing about the gravity of each kind of crime except that they shouldn't be the same.
The only reason "sexual assault" covers what allegedly took place is because the expression was redefined in the first place to cover two separate things, which is exactly the problem. Stallman called not for redefining "sexual assault" as you put it, but for undoing a redefinition that puts it at odds with how those terms are conventionally used and causes unnecessary confusion.
And "free software" makes people believe that the software costs nothing, but Stallman's position there isn't to suggest that we should always use a different term. Why not?
I agree the expression "Free Software" has the same problem. However the alternatives also have some problems. He discusses that here: https://www.gnu.org/philosophy/open-source-misses-the-point....
(I'm sure you've already read it, but it's always worth a re-read)
I'm partial to the term "Libre" myself, though in this case it doesn't matter much, since it's a concept that people aren't familiar with anyway and you have to explain it no matter what word you use.
The only reason "sexual assault" covers what allegedly took place is because the expression was redefined in the first place to cover two separate things, which is exactly the problem. Stallman called not for redefining "sexual assault" as you put it, but for undoing a redefinition that puts it at odds with how those terms are conventionally used and causes unnecessary confusion.