Why have historians called the yrian Empire of the first millennium B.C. the first true empire?

Because it seems to be the first one where there was direct rule over foreign peoples. Before then, the Egyptians and others had left native rulers in place, just imposed an obligation to pay tribute upon them. Further towards the outskirts of their empire the yrians still used this (as with Judah, for example) but there was a large swathe of conquered directly ruled territory between that area and the yrian heartland.