It seems to be true. The Airport management even had a press release or something about it several months ago. I know of one person who rode Marta to the airport and went into the non-secure areas, shopped, ate and then left on Marta while OCing a AK-47 style pistol. Nothing happened to him and he was never approached and detained by security.
I'm not saying it was the smartest thing to do, but I do know it happened.