As @Jak Nieuwenhuis
said, the high volume bodyweight training is something I often do because it works well on me.
First off, it is important to remember that H. Walker follows a strict diet (vegetarian + warrior diet if I remember well). Plus, this high rep training is in addition to his specific training (nowadays MMA). Then, I guess that getting this kind of physique is his is a combination of these items. Finally, some folks respond better to high rep training than low rep training, and vice versa. This is very "personal". I never got sore or neither "tired" doing multiple rep training. However, a true strength training (excepted GTG) will fatigue me pretty fast.
I built up volume until reaching 1500 push ups a day and "only" 1000 sit ups. Regarding the leg, I walked about 10km a day (no car, just daily life) and ruck with 20kg the WE. I also paired the push ups with 2 or 3 sets of pull ups, close to failure, to work on agonist and antagonist pairs. I also do a few "full tension" back bridges after each sets of sit ups. That work I work the spine in flexion and extention.
Regarding the results, with a balanced diet, I finally got pretty visible abs ! Regarding strength, it permits to maintain my "top moves" (front lever, bent arm planche, pistols @24kg for reps, OA chin, free standing HSPU for reps, feet elevated OAOL PUs for reps).
Of course, this high rep training may not be optimal to really gain strength. However, sure it maintains maximal strength once you reach a certain "critical" volume. I noticed that for my body, I have a better transfer from "high rep easy variation" to "low rep hard variation" than the contrary. This means that I transfer better strength-endurance to max strength, than max strength to strength-endurance. I am also far more powerful training that way. M. Tyson also use massive basic calisthenics moves.
Also, these sets during the day build a huge mental toughness and discipline. They become something natural which is easily incorporating in the daily routines (such as a shower, brushing the teeth, and so on).
Below are some other protocols:
Proper Situps - Pass Situps Tests - Curlups, Crunches Tests From Stew Smith CSCS
PT Secrets - The Pushup Push: Double Pushups in 14 Days (FREE) - Stew Smith Fitness Ace the PFT - Preparing Americans for Military,Special Ops, Police, and Fire Fighting Professions
Pullup Push - Double Your Pullups in Two Weeks! (FREE) - Stew Smith Fitness Ace the PFT - Preparing Americans for Military,Special Ops, Police, and Fire Fighting Professions