Your right, however, that is to a point and to a limit. With no possibility to increase load with bodyweight only exercises. Progress may soon be stunted or never really begin at all. For some people many body weight exercises are never enough load to begin with.
I'd say that these guys did pretty well strength/size-wise by using body weight exercises:
Lets say, for example he can complete 30-40 push ups nonstop already. Would you suggest to him to keep at that to increase his pectorals in size? Of course not.
Try to perform even 5 one-armed pushups, and then we'll go from there. I can probably come up with at least 10 pushup progressions off of the top of my head if you needed them.
If ones goal is to gain size. There is no point to sticking with bodyweight only if weights are available.
Improved co-contractions and joint stabilization, increased synergies and neural patterns, decreased cost and space demands, certain functional demands, rehab/prehab, just to name a few. This isn't an argument about "weights vs. body weight," but just to point out that bodyweight exercises not only can have a place in a routine, but often times they
should. Will it be optimal to have a routine 100% bodyweight if you're looking to gain strength/size for a relatively advanced lifter? Probably not...but it doesn't mean that it can't be done...you just have to be more creative.
In a situation where a person couldn't even complete a single push up. Than yes, this would be a situation where size might develop beyond a negligible extent. However, I would assume the neurological aspect of resistance training would be the primary cause of this "weakness". In which that would be corrected in a manner of weeks.
Neuromuscular and physiological contributions to strength go hand-in-hand. You can discuss one vs the other in conversation, but that doesn't really happen functionally in the body, so regardless of what the initial "cause" of the weakness, the body will produce adaptations in both areas. At times, one area is more responsible than the other in producing strength increases, but both work together and are the stimulus for improvement in the other, so again it doesn't really work the way in which you describe.
Meaning, the muscle fibers them selfs were always able to handle the load. The neuromuscular inefficiency is what lead to the poor performance. So, there is never enough load to begin with to encourage growth in the average person. When the body is used as the only resistance.
Of course, thats just my thoughts on it...
You're describing cortical adaptations in a raw beginner, and a raw beginner doesn't go from the inability to be able to perform even one pushup to 15+ in a matter of weeks. The adaptation curve just isn't that steep.