I agree. That is certainly the argument I'd make. If the word of the law is correct in the article, then it is not illegal to sell a firearm, but only illegal to sell it to an improper person. A cop would be a proper person, even if he was pretending to be a criminal. Unlike drug distribution, it does not appear to be a law against intent.
But it can't be that easy, can it? It's not like this law was hastily slapped together where there would be huge loopholes and mistakes in it, like... oh, I don't know... forgetting to exempt law enforcement from the high-cap magazine ban.
You're forgetting the part where it didn't get rushed through an emergency meeting, which made it be voted upon immediately instead of allowing a 3 day review.