"Buddy, before you kidnapped my best friend I had no idea magic existed. Whatever we did, we did accidentally, on pure" - Oh, this is a bad idea. - "God-given talent! Now give me back my friend and butt the fuck out of my life unless you want to see what I can do when I'm actually trying!"

Is this pure bravado? Probably. It's not like Arthur knows what he can do any better than this other guy does.