I think I must be misunderstanding you, then. Unless I've missed recent IP case law, which is always possible, I really can't see how?
Looking at it logically, Epic licensed the right to use IP names/likenesses/sound clips within the Fortnite ecosystem. That's the extent of the license, so striking a deal to bring Fortnitemares to HHN doesn't convey a sublicense to use those IPs at HHN. Otherwise, we'd have Disney villains running around the scarezone.
Universal would still have to turn around and negotiate licensing name/likeness/dramatic performance rights from Fun World/Paramount/Spyglass to have Ghostface running around, for instance.