I’d have to check the book again to say how strictly the schedules are suggested to be followed, however it is worth mentioning that everything in the book is a suggestion. There are many ways to work your way up to the master steps of the big 6. I can do 3/6 master steps and for all 3 I used different assistance exercises to build up to them than the ones in the book. For the other 2 that I believe I will be able to build up to, I expect this trend to continue.
Use the book as a reference, but as any other resource, it is simply one of many from which I would encourage you to draw your information in designing your training regimen.