I think Simon was genuinely sorry for his first letter, but the apology did come off as entirely self-serving. For all those excoriating him, he is generally a decent person but one of his less pleasant qualities is a tendency to be judgmental.
River responded very nicely that she forgave him long ago and wished him and his family well, but please do not contact her again. It was pretty much a textbook-perfect response. I'm not sure I would have bothered.