The other auto-loading rifle in the book, the Gewehr 43 has it as well, so I don't think it's a typo. I would assume they have the auto trait for the higher rate of fire they can put down range and that that would equate to a 6" FZ rather than a 3".
Personally, I love how the little addition of a trait or an extra piercing/1 can totally change how a weapon is used in the game. Feels very positive to me, rather than the exceptions based rules sets for some other games where every unit/weapon breaks the rules somehow. (and there are a few of those in WaW to be fair, but just a few)