The reason for that warning in the book is that we were targeting climbers, most of whom we assumed had pretty much zero experience with controlling the intensity of training and were already training too hard. So we took a very conservative approach to explaining this stuff.
Zone 2 training will have a more powerful aerobic training effect than will Zone 1.
To know if you are in need of adding more Z2 or sticking with more Z1 you can do a simple test for yourself. Conduct the anaerobic threshold test described here: https://uphillathlete.com/diy-anaerobic-test/. If the average HR you determine in this test is more than 10% above your AeT (136) meaning higher than 150 you can do most of your aerobic training in Z2 because you have a pretty significant aerobic deficiency and will see bigger gains that way. If your AnT is less than 10% above your AeT then chances are if you run too much in Z2 you will find that your leg muscles don’t recovery well over night and you’ll have to spend more time in Z1 or at a recovery pace.