That's my point. Lumping 9-5 in with "not too bright" gives the false association of "working normal hours" with "not being productive". There are plenty of ambitious, bright workers who manage to get it done in normal hours.
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.