I agree with ChippyGaming on all except that I'm almost certain it *is* an invasion event, but themed around Dungeon Defenders. I cannot believe that a portal that lets you go to another world be a "minor" release (i.e. 1.3.x). Such a change would necessitate far reaching changes - you'd need new biomes with new blocks etc, new save structures to save the "parallel world" data to, as well as lots of new mobs, etc. Also, it would be such a huge feature that tying it to a game cross over would seem like an anticlimatic way of launching it.
A themed invasion event, on the other hand, needs maybe a dozen enemies and some things for them to drop. It would match perfectly with other themed invasions, like martian madness - i.e. monsters from another world invading. The difference here might be that a number of portals spawn, which in turn spawn the mobs, instead of having them spawn off screen.
Edit: What clinched it for me was the portal in the Dungeon Defenders spoiler clearly has the moon lord carved into the ritual circle the portal is on.
As an aside, I'd expect DD players would be most likely to get the same thing - a portal that spawns a new set of enemies based on those in Terraria.