I have not (and will not) see the film, but from what he has written it appears as though that they broke the law. I have not disputed that in any way.
That being said, the author goes to great lengths to explain the pertinent federal law concerning out of state firearms purchases. My point is that despite the effort, his statements are not totally correct and there is no reason they shouldn't be. Since a rifle (the bushmaster) was one of the purchases, and that particular rifle is under fire for being some kind of kid killing machine, I think an accurate statement regarding the purchase of rifles out state would make the article better.