the book is by ultra-marathon runner, so yes you can be, but if you are just "casual" runner, you probably wanna be healthy first. the point is you need to teach your body to burn fat and intensify the training slowly by getting "fitter", meaning you will run faster in the same HR zone as before - the zone that is indicator of your body burning fat.

Exactly. No one should pick between slow burn and fast burn. It's much better to incorporate both into your exercise at different times of the week , to address different areas of fitness and to train both slow and fast twitch muscles.