Also the converse is true - doing something as often as every day is effective if done right. Look at anyone who works in heavy labor, like farmers or construction workers or movers. They tend to be really big and strong from all that daily work. Rehab in hospitals is also done daily to get you back on your feet. Also check out the calves of overweight people or who used to be overweight. They never walked to failure, not even close, they just carried more weight than you on a daily basis.
If HIIT is extremely high intensity and very low frequency, then the opposite works too - mild intensity, high frequency. You can always juggle volume/intensity/frequency. It may not have the same impact on your mitochondrial DNA like super high intensity does, but if you’re after muscle strength and size and the health benefits that come with such development, there are many ways to get there.
It depends on your goals, your schedule, and your personal preferences. But for example, on the extreme opposite end from this book, doing one set of push-ups, pull-ups, and squats (or whatever exercises you like to accomplish the same effect) at a regular speed and like 1-2 reps shy of failure every day will develop strength and size very effectively. It can be accomplished 3-5 minutes before you shower and go to work. Feel free to take days off whenever, always listen to your body.
What I found over the years is consistency trumps all other variables. People give up after weeks or months, and then resume again next New Years resolution or whatever just to start over. Just pick whatever intensity/frequency/volume combination you can stick with long term, don’t overthink it, make sure your form is good, and cover the major muscle groups with compound exercises like the big 5 in the book.
The 3 other variables that are just as important as training is eating, sleeping, and stress management. If you can’t get those right, you won’t be recovering effectively for your next training session and may end up hurting yourself more than helping. On the up side, lifting heavy things sure stimulates the appetite and makes that steak even more enjoyable and helps you fall asleep easier and more deeply if you don’t get in your own way with bright screens and light before bed.