The quote and he thought it must be wisdom, this new indifference to the incidentals he'd been offered and had enjoyed to me is interpreted as wisdom referring to Severus's new feelings of "this new indifference" to the shop-keeper's offers and how much he enjoyed them. The word "wisdom" is not referring to "Harry" in the structure of the sentence, it's referring to "new indifference". So knowledge of his "new indifference" was not how Severus had behaved previously in similar situations. Severus's new wisdom tells me that before he was more obsessed when he engaged in such behavior. It was just his new wisdom in realizing that he was no longer obsessed with his affairs and could still enjoy them without becoming attached to the people whom he was having the affairs with. It didn't mean that he would never have one again. He didn't realize any sort of wisdom in what he had to lose with Harry because he already knew and understood his feelings for Harry. It was only the nature of his feelings toward the dalliances that had changed. He could now have them and enjoy them without being obsessed with the person involved, and in his mind still care for Harry.
Also there was another sentence about Snape saying how he "wore his mask well" when he was a spy. He still wears his mask well. He has not changed his behavior in terms of going out and remaining secretive when it suits him. Severus is hiding truths from Harry if it benefits himself, which isn't good in any relationship.
Amazing how we can see the same thing so differently, yet I can understand what you're seeing and we can still disagree :)
I do agree that the infidelity prompt was nailed perfectly.