I don't know if he's a purely a WCO coach or not. It may simply be a case of Sherman not screwing up something that's not broken (something too many coaches do in the situation he was in). He appears to be rather new to the WCO, only being involved in that system since joining the Packers in 1997 (interestingly as a TE coach and assistant OL coach). Prior to that, Sherman was an OL coach dating all the way back to 1983 with Tulane, Holy Cross, UCLA, and his most recent job prior to joining the Packers was at Texas A&M, again, as an OL coach.
I find his experience at Texas A&M the most interesting and the most indicative of the fact that he probably is NOT a WCO only type of coach. That's not exactly a passing team, but he coached there for almost a decade before joining the Packers. Most likely, the WCO is simply something he picked up under Holmgren, but he's probably got experience coaching in a lot of different systems.