Read it again. RMS wasn't defending statutory rape, he only wrote to not use the term "sexual assault" when there was no evidence of violence being involved, because words have actual meanings and using wrong words confuses the issue.
"sexual assault" doesn't require violence.
The problem is with the ambiguity of the term. From a legal perspective, "sexual assault" doesn't require violence but for ordinary people, use of the term implies some sort of attack or aggression. It's only a few years ago that I learned from online discussions that in some jurisdictions the term "assault" has a specific legal meaning that does not imply violence.
Doesn't matter if the accused event happened or not. Defending the hypothetical in that manner gets you fired.