Well, I'm not sure that he said, "union." What he definitely meant is Davis-Bacon wage rates, which are supposed to be an average of local area wages paid, including union and non-union. In actuality, local union scale is most often paid. I'm not sure why it plays out this way, but it does, and it has no union membership required.
It's possible that non-union contractors took to paying scale to avoid having union business agents on the job promoting union membership. My brother did this in Michigan many years ago, actually joined the union himself, just so his workers did not have to. If a guy is making union scale, he's a lot less likely to want to become a member.
Many state-only projects also require paying scale, most likely to avoid strikes, and to lure non-union contractors to bid.