I think you're correct it's just air being mixed in not nitrogen, specifically. Stores like Party City started pushing things like arches and stems and other things that hold up balloons up rather than float. It is a lot cheaper to sell a larger quantity of balloons filled with air than a few helium-filled balloons. They also switched their floating gas mix to 60/40 to reduce costs (except for mylar balloons which are heavier and require 100 to float). That's how I remember it from buying balloons for birthday parties a few years ago.