I was wondering how Dbx and toadswoot (i'm sure there's multiple other users who can do this, they're just the first that come to mind) are able to mash their selected b-button so quickly, do they use a special controller?

You can mash fast by holding your forearm steady and tensing your hand so that you "vibrate" over the button. It's quite tiring but you don't actually need to mash fast in this game.

Using a special controller with additional functionality, like one with a turbo mod, is banned.

Small add-on, but you'll also notice some of the bottons they put their items. ToadsWoot has said before that the reason sometimes he puts his sword on A is because he's much better and faster at mashing with it.

But as Kitty said, there's not too much need for fast mashing, so don't feel you need to.