Before saying the defense is "for real" I'd first need to know what they mean by it?
I think the defense is pretty good (not great) as pass rushing and the DBs are pretty good at covering which is what the defense is based around attacking the passing game.
Is this defense dominant? I'd so no... but can it be dominant? I'd say yes.
To me this defense seems to be based around pressure and playing from ahead (which normally makes the behind team pass the ball more) would fit right into the hands of what the defense seems to be doing... which could potentially perfectly match the Packers offensive style... with Rodgers at the helm, as the other team tries to play keep up with Rodgers.
This defense playing against a run first team like say the Seahawks or Cowboys could cause big problems.... or maybe if a team got a big lead... but I think they would toughen up and help keep them in it for the most part like they did against the Lions. I think a power run first offense with a decently passing game on the side might beat the defense though.