Injectors are spraying nearly the same fuel amounts on the MT vs AT at same speed when cruising on the hwy.. Even if the AT is turning lower rpms.. has nothing to do with lower rpms.. Only way RPM matters is the window your ECU looks for when throttle is at what % and what rpm you're at.

Honda isn't dumb, they aren't going to let the MT rev that much higher on the highway and let it effect the mpg.