Reading that letter it seems that Jon was being accused of... nothing in particular? I'm not even sure what he could refute. There's no accusations of consent violations. There's really only the one phrase - "sexually harass and victimize women" - but without examples that just sounds like a pot shot. Especially given that they identify a "systematic pattern", which is apparently a pattern with no specific examples of wrongdoing.
And don't get me wrong, I'm strongly inclined to believe women and I generally distrust men. Especially when it comes to their interactions with women. And I believe these women probably had plenty of valid complaints, in part because I know very well how aggressive, oblivious and entitled a lot of men can be, and how many "normal" interactions between men and women do involve consent violations, if not assault or worse. So given what I know about the dangers women face routinely, and the vague and mild allegations in this letter, I'd guess like the biggest crime he committed was being another guy who's god awful at dealing with women, dating and sex.
Unless there is more that I don't know about.