I think the uses of "sloth" and "lazy" here are quite importantly different. The idea of working smart by being lazy, really means (at least to me) coming up with a better solution than the obvious hard work, time consuming approach, then going off and doing something better with the time that you have saved. Sloth implies to me sitting around doing nothing.

So: general sloth=bad;lazy approach to some tasks=good when it frees your time to do more important stuff.