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?
The legal definition is wide enough to encompass a broad range of acts that people have differing moral intuitions about (even if they are all bad acts, some are markedly more heinous in a lot of people's minds). That leads to moral vagueness. Pointing to a legal definition doesn't change that.
If I went around calling you a killer, because you kill insects, or a criminal, because you had an alcoholic drink when underage, or a producer of child pornography, because you sexted your girlfriend a photo of yourself when you were 17, and then pointed to a dictionary or legal definition to show that I was technically correct, would you not still object that I'm using terms that are overly vague as to the morality of what you have done?
There are two issues here, one being Stallman's hesitance regarding the use of the word "child" depending on which country this all took place in, and two being Stallman's comment that there may have not been any coercion at all.
While it's vanishingly unlikely the latter was the case, to vilify Stallman for these statements instead of making scientific inquiry is absurdly reactionary and unempathetic towards someone who is very naturally trying to make rationalizations towards the behavior of someone whom they hold in high regard, as is typical in during the doubting stage of grief.