To me, ”They fly now” would have worked much better if only 3PO said it. He delivers it in keeping with who he was when he was in the asteroid field from Empire.
Not to mention that Finn should know about such troopers, and Poe has encountered jetpack troopers in his Marvel (Disney) comic.
Troopers with jetpacks is far from a new thing.
So much for Disney respecting the EU.
It’s this, more than the humor in and of itself, which has fans ripping it a new one.