He is just trying to create public conversation about clarification of the term "sexual assault" when it comes to murky areas of consent. If it came out tomorrow that Minsky indeed raped an underage child, Stallman wouldn't deny the truth. The truth is all he's after.
34 U.S. Code § 12291 (29) - "The term “sexual assault” means any nonconsensual sexual act proscribed by Federal, tribal, or State law, including when the victim lacks capacity to consent." - https://www.law.cornell.edu/uscode/text/34/12291#a_27 . It does not require 'force or violence', as Stallman implies.
Stallman stated: "Whatever conduct you want to criticize, you should describe it with a specific term that avoids moral vagueness about the nature of the criticism."
How is "sexual assault" not the specific term for sex with a coerced minor? Where is the moral vagueness in the accusation?