>>715293260A. It's characters you're very familiar with, you've probably heard every single thing these characters have ever said.
B. Characters you're used to seeing multiple variations of at the same time already. A few more doesn't feel too far out of place
C. TF2 is already very silly and cranking that up another few degrees also doesn't feel out of place.
Altogether, you get humorously animated, twisted versions of characters you already love, having their words stitched together. But they ultimately still end up feeling like the same people because their personalities and relationships to each other are simply so deeply ingrained that they come through no matter what you do to them.
Which, of course, ultimately follows on from a very particular lineage of YTP and Garry's Mod comedy for which you are already fond.