Gotcha. I would agree that you were invited specifically to justify giving you co-authorship. As evidenced by the existence of our conversation right now, exac

Gotcha. I would agree that you were invited specifically to justify giving you co-authorship. As evidenced by the existence of our conversation right now, exactly what level of involvement justifies co-authorship is fuzzy at the margins; whatever their personal views, they were probably doing you a favor by moving your involvement from the fuzzy zone into the hard-to-argue-with zone. Even if everyone directly involved in the paper thinks "being an intern host justifies authorship" the presence of some senior institutional figure who disagrees could cause problems for you.