I think what you're missing is that "mashing" implies "pushing hard". If the same person is able to "spin" in that same gear on the same terrain, then if they slow down and do it slowly, they're not really mashing. If I go pedal my single speed around at 5 mph on level ground, that's not mashing. If I get on a hill where I can only go up it at 5 mph, then I don't really have a choice of speeds, either, so spinning isn't an option.