What's the big deal? Colt made a 7.62x39mm AR-15. Oly still does. This one takes AK magazines. Forgive me, but it seems this rifle's maker stumbled upon the obvious.
7.62x39mm is superior to either 5.56 or 5.45 for suppressed use, however. A .223 caliber or .21 caliber (respectively) bullet can only be made so heavy. For suppressed use, you want the heaviest bullet you can, since you have a velocity limit of around 1000 feet per second (so as to avoid the sonic crack).
A bigger bullet can be made heavier. A 250gr. bullet @ 1000fps is better than a 100gr. one.