I think intensity here is a proxy for metabolic equivalent task (MET). In a lot of literature surrounding recommendations for aerobic activity, it’s generally a

I think intensity here is a proxy for metabolic equivalent task (MET). In a lot of literature surrounding recommendations for aerobic activity, it’s generally acknowledged that you need to be around about 3 MET before the cardio activity counts towards cardio health. 2.5 mph walking on flat ground is a benchmark for 3 MET. (This agrees with the findings of this study in that the low-MET slow walking wasn’t correlated as positively with health.) To extend to your point about more intense slower walking, that would still be a higher MET value than slow walking on flat pavement.