Neither is incorrect. What is incorrect is the idea that the bias is some super critical precise adjustment. You can bias an amp so hot the tubes get red hot from excessive current. What we call red-plating. And you can bias an amp so cold the crossover distortion is too high to listen to. But really, in between those extremes, the amp doesn't really care. You may prefer the sound at some particular setting, but that is a matter of taste, not correctness.
My reaction to those methods you found: Nomad bias means they liked the performance at 75ma, assuming average tubes, regardless of what the dissipation worked out to. The vintage method sounds like an invitation to use 70% or some other target you like as the goal, regardless of the current needed to achieve it. IN general, guys bias their amps a lot hotter than amp makers intend. For example Fender didn't ship amps biased to 70%. SO Carvin is telling you to either set it for their level, or pick a dissipation target and use that. That acounts for the one being hotter than the other.
I don't recall the model, but Carvin had one a while back that was adjusted by mains current draw.