She is just calling it that like it is the common way to refer to it, but it isn't and is in fact made up for the show. If she had said "Buenos Airs is the big apple" then maybe, but the awkwardness of the way it's just forced into the musical dialogue like it's a known and accepted and used phrasing is very strange.
I think he was just happy with what he considered to be the cleverness of the b and a... "BE A part of B.A., Buenos Aires, Big Apple".
