I think the assumption there was that, if everything but the case is gen, pretty much any decent rep case would be undetectable from normal viewing distances
Around the same time that thread began on VRF, there was a thread here on rwg where someone replaced the case from a standard GMT rep with a gen. My 1st thought was why would they do that since everything else in the watch -- dial, hands, crown, bracelet -- was easily identifiable as standard rep fare. Cases that most better quality reps come with are the least likely part to be identified as a rep, so a gen case is probably 1 of the least cost-effective places to put your money since upgrades there result in the least improvement. That said, once you arrive at the point where everything else in the watch is gen, you would be crazy not to swap the case out for a gen.