Not too sure who this Doug character is, but to me it appears he is a bit clueless (I would say a complete F&^*WIT but that might come across a bit strong).
Ignore this program completely, it is completely flawed in every aspect. This is the type of program that we use as an example of what not to do. Unfortuentely the internet is full of this type of thing, that wastes peoples time and effort.
I'll keep this a little short or I'll be here all night.
The volume for each major muscle group is ridiculous. Who ever designed this does not understand the overlap of muscle groups in different exercises. Chest has 3 exercises, 2 of which use triceps quite heavily. Then the day after you have flogged both triceps and pecs, but you flog the same muscle groups again with a tricep workout, which of 3 exercises, 2 are highly dependant on your chest muscles. So in effect, you are training the same muscle groups on concurrent days, which the second sessions offers nothing and limits improvements from the first day.
EDIT I think i read the program wrong, chest and tri's were on the same day. Although the following day does work the same major muscle groups nether the less. Also, 1 week between sessions, ie legs, is more of a maintenance time frame, want to train muscle 3-4 days apart.
After you exercise a muscle, remembering that compound movements use many muscle groups, you need to rest that muscle to let it recover and grow. Muscles grow because you stimulate them into create mRNA that is the blueprint for developing bigger stronger muscles, this peaks around 48 hours. If you train the same muscle group before 48 hours, you depress mRNA and the muscle does not get bigger.
Any exercise rep range of 'to failure' is poor programing (depending on the circumstance), unless it is low reps. if you can do 20-30 reps of something before you fail, the load is way to light.
As far as needed to change exercises regularly, sometimes called blitzing or shocking the system, there is no need to do this, evidence actually suggests this will limit for overall improvements in a long period of time vs using the same exercises. Olympic lifters and power lifters NEVER change exercises, they are impressively strong and large humans.
There is a lot more wrong here, its like its designed to be impressively complex, but fails in every major way.