I don't like to create rules. I'm not a one system/style guy. I use the tools I need to get the job done. So I can't have only one set of principles.
I believe that the best training is the one that fits your psychological/neurological profile while still providing your the stimulus you need.
Some people respond better to a very high frequency, some to moderate frequency and some might even need a low frequency. Some respond great to low reps even for muscle growth and others need moderate or higher reps. Some need a lot of exercise variations, others do better on a minimalist approach, etc.
IN UNDERSTAND THE NEED TO CREATE FIXED, UNIVERSAL RULES ... or to put people in boxes... it is reassuring to hold on to something that will always be true. BUT the fact is that while there is a science to training, there is also an art... and the art of training is selecting the best type of training for each individual. For that reason, universal principles don't work. Of course the title of some articles might be misleading but it is an editorial choice to make the message easier to get across.
That's why in some people I will use a low volume and very high frequency, with others I use a lower frequency (each muscle 2 times per week) and moderate volume. In some I even use a fairly high volume, even if they are natural (but these are normally genetically gifted athletes).
Some coaches are great at designing a specific system and stick with it. My "skill set" is fixing problems and finding the best type of training for each person.