I agree it seems like a concern fad. I talked about it once with my brilliant MIT-educated 20 year Navy submariner brother-in-law and he didn't commit one way or the other but did say submarines have CO2 in the low thousands.

You'd think (hope) if there was a big effect here on performance, the relatively cheap/easy solution of maintaining lower CO2 would be standard. I know people think of the military as dumb grunts who you don't want to think, but he was one of the four department chiefs onboard (Weapons, Nav, I forget the others) and they have pretty substantial responsibility to make decisions on their own.