Yes, I was unfamiliar as I think the majority of audiences ever seeing this musical or listening to it would be. And I don't think most people would think of Nathan as a specifically Jewish character who didn't know this, and many outside nyc would confuse his more Jewish comedic lines as New Yorker speak more than Jewish speak. I know the term no-good-nick, i did not know it was Jewish. I do not know "zo nu?" I think my experience is very common for people listening to or seeing this musical, and likely would have been even when it premiered. It also truly doesn't help that nu is a homonym for the much more well know English word the English speaking audience uses daily, new. It also doesn't help that Nathan being Jewish is written in more as a color of his character and not a context we need to have like Tevye for example.
No, actually it really does make sense as "so new?" On that we do not agree.