I think a lot of this is psychological. If a doctor needs to tell you to work out, she needs to say words that will have the best effect. For many, that is walking. For others, she'll suggest zone two jogging or biking type stuff. For those that already jog or bike, perhaps she'll mention adding in some more strenuous exercise.

So even if the article is correct, zone two is still lets say 90% as good, while being a lot easier for many folks to commit to.

Also, zone two is "Low intensity steady state", meaning a person could sustain it all day. By definition, you can't sustain the higher zones all day. So if you want to train a lot, your only choice is zone two.