That "rumor" was published in the linked news article, covering the arrest. That is Ferguson's official word to the police, that the weapons were inherited, and his family has restated this to everyone who's asked. I'm sure that'll be tested if this goes to court.
He didn't simply go out and buy a gun. Around a dozen were confiscated, which sounds much more like an inheritance. I'm willing to buy this part of the story, it sounds very plausible. Though why the weapons weren't simply given to his parents or another relative, is really beyond me. Sounds like the family is pretty bargain basement stupid.