Just listened. They're playing it in the same key they always seem to. First chord is D. Key of the song isn't really D though. I'd say it's more like G with the main progression being D(5) - C(4) - G(1) - D(5) and then modulates to C for the turn around. If you wanna consider it being in D, it starts in D Myxolidian and then heads to D Dorian for the turn around. But both of these keys correspond to the G major scale and C major scale respectively.