I don't care if they openly say ERA is all that matters or not. They are clearly basing their entire opinion of McCarthy on his ERA (and perhaps W-L). Consciously or not, it's happening.
If he had a 3.00 ERA but looked exactly the same when those people watched him and he had the exact same peripherals, the non-stats crowd would be all over him and would be pissed that the Yankees acquired him so cheaply.