I think it matters more if you are cognitively present for the exercise, rather than the intensity of the exercise. It doesn't matter if you go fast or slow, it only matters that you are doing something you're not used to and as a result actively think about it when you are doing it.

For example, writing with your left hand if you are right handed may be an equally effective method of achieving similar cognitive results.

One well worn piece of advice from health fitness trainers has always been to vary up your routine. I think this is the key to making sure you work out different muscle groups, but also a way to keep your mind engaged.

My hunch is that this is partially what's going on in the study.