In this context, 9-to-5 most likely means "stops thinking about the job when not on work hours". I doubt it's meant to imply they aren't smart, but they aren't necessarily as ambitious and motivated (or they've determined the cost-benefit ratio of out of work hours work is not worth it). People that go above and beyond in an organization are really who you need to worry about if that organization is your enemy, as they are harder to predict.