Epstein was an awful person who did awful things and should have spent the rest of his life in prison. But that doesn't mean that every specific attempt to put him in prison was legally sound, and it doesn't mean that anyone whose actions contributed to that specific attempt's failure is somehow complicit in Epstein's evils.
It seems to me that it's a positively good thing that when someone is accused of something awful, they can still get vigorous legal representation, and that the people providing that vigorous legal representation can solicit opinions from experts that they hope will back them up. Because 1. sometimes people who are accused of awful things turn out not to have done them, and 2. when they did do them we want the legal proceedings establishing that they did to be as watertight as possible.
In this particular case, the issue wasn't "did Epstein do X or not?" but "are the things Epstein did covered by this specific law or not?". It seems that they weren't. Again, I think it's a positively good thing that when someone is accused of something awful we only get to use a law against them if that law really covers what they did. Because if there isn't a general principle that you only get punished for things that are actually against the actual law then what we have isn't laws, it's excuses for the powerful to punish whoever they feel like punishing.
To be clear, it looks as fishy as all hell to me that Epstein didn't get a much harsher sentence in the 2006-2008 case. It looks, in fact, like some sort of corruption or cover-up. But in that case, I don't see how Pinker's letter had anything much to do with it. And, again, so far as I can tell what Pinker wrote was simply correct, and when he wrote it he didn't know it was for Epstein's case.