I dunno if I agree about the walk being "better" than the 5K exactly? I think you would be excluded from the study which appears to be studying the impact on sedentary people and 20 minutes is a pretty brisk 5K < 7 minutes/ mile! It seems as if they are concluding that the benefit was the extra effort to walk and stand made by the non-exercise group. It also appears that the results seem pretty close and, to my eye, may be within a margin of error that might wash out some of the data a bit.
Still, I don't disagree that mixing in any activity, walking, standing, etc. is a good thing however I would try to do that *and* a workout. I think there's some benefit to saying "this is the time I'm going to workout" and getting into the flow of it, untangling yourself from the strains of ordinary life and, for us, beating our BG into line so we can hit the road, trail, treadmill, pool or whatever. That 20 minutes of more than just standing can be very healthy in ways beyond CHO/BG/etc.