Don’t judge a system by the physique of the person promoting it.
The authors warn against following workout advice from somebody who is clearly out of shape or does not care for lifting at all, but at the same time not to choose a workout program just because the person promoting it has the best physique or the heaviest bench press. All that that tells you is that their system works for them.
Most practitioners should be in at least decent shape. How somebody looks or how much they can lift has a lot to do with their genetics and discipline. Their program very well may work for you. But, it may not. Somebody who may not be as large could possibly have the best program for you, so you will want to keep an open mind and experiment.
It is hard not to judge a book by its cover, though. It’s the first thing that you see and all that you initially have to go on for whether something is worthwhile. My process is to usually read and compare different workouts and try to see what fits into my schedule and my experience level. I will then try it for 3 weeks or a month. If I like the workout, then I will continue with the next phase until I’m ready to try something new or reach a point in my running season where I have different goals. If I do not care for something, then I will move on.