The appearance of Ant Man only makes sense in the context of the airport scene. I was mystified until they get to him reversing things and going to Giant-Man size. At that point, it becomes clear that it's pure fan-service, but weirdly gripping in that context: they were kinda-sorta recreating the death-of-Goliath sequence from the comics, which is one of the turning points in the original story. Far as I can tell, the whole point was to mess with the heads of the hardcore comics geeks...