Two questions to ask here. He's not as strong against righties as he is against lefties, but he's not competing against mirror double, he's competing against Scutaro, who isn't a very good offensive player. Also Scutaro is also righthanded, and has a slight platoon weakness of his own that puts his healthy .715 v. right in the same universe as Lowrie's injury-exaggerated .660 or so against right. And the comparison between Lowrie and Scutaro against lefthanders is frankly embarrassing.
Lowrie had a .823 OPS this year v. right this year, probably his first healthy sample since midseason 08, which was his rookie campaign anyway. SSS is playing a role, but "can't" is an overstatement. Playing down the significance of 08 and 09 because of the wrist injury, which we all know affects switch hitters' ability to hit differently based on which side of the box they're standing in, I'd expect him to have something of a platoon weakness, but I'd also expect him to put up about a .720-.750 OPS against his weak side which is about as much as Scutaro can do against his strong side.