It's great that American Sign Language has been used in this video. Not only that, it's apparent that this isn's just an 'English translation' as many attempts at this kind of thing are. It seems like genuine sign language to me, with all the nuances of the language (except, maybe, facial expression.)
Apparently Johnny and Natalie have signed some parts of it wrong - swapping the signs 'appear' for 'tampon' and 'enemy' instead of 'valentine.' I don't know about this specifically as I am not an ASL (American Sign Language) user. However, although those gaffe's might be there, I don't think they matter too much in the long run. The fact that sign is on such a prominent video, that it is genuine sign language and not 'English-sign' matters more than a few small articulation errors, to me.
And, well if Johnny would like any private NZSL lessons.... I would be more than willing ;)
(Just kidding, Matt!)