I don't know where people are getting the info that it's heart rate related, but I think it's not.
My wife did another workout and her average HR was > 130 this time. Still didn't count it.
I think it's related to pace! Anything under 3mph seems to not count. It counts as a workout, but not the exercise ring.
(hers was 2.5 mph average, but on a big incline.....)
Maybe it SHOULD use HR, but I think it's depending on pace.
In my original example, I was doing nearly 4.25 mph walking fast