I started a new thread with your topic.
We used % of AnT HR because in this plan you’ll be doing more intensity. Normally we don’t add much intensity until one has had a substantial aerobic base building period. But in this plan, because of both the shorten of the plan and its time crunched nature, we move to more intense workouts whether the person is ready for them or not. We do this because this will show good short term gains. But we are also careful to explain that this method is a compromise for long term development. So, yes we do this because 6 hours/week of Z1-2 training is not going to give much aerobic training effect.