How would you know that it isn't and has never been said by anyone?
But it wouldn't be "So, new?" I agree that isn't an expression and no on has say it - i'm not why you wrote it that way. It's "So new?" It seems like you are trying to make it more absurd that anyone would think that was the expression than it actually is.
And sometimes when you use an idiomatic expression that the majority of people listening to a cast recording would never know was one, and that expression including a homonym, then everyone is going to assume there either IS an expression such as the one the character sounds like he's saying, or that it was created for this and sounds good and makes sense. Even as "so new?" it sounds like a very Jewish thing to say (i am Jewish and was raised by NYC Jews).