Everything you say is true, except that it ignores several key concepts, and I'll focus on pitching in my example. Today's pitchers, you can see most of them are max effort to get the velocity they get. Even watching old footage of games you can tell the pitchers then were not. Then you also have to take into account that IF Walter Johnson was throwing 100mph consistently as Jim suggests, he would have been doing so every game from start to finish? There wasn't 5 man rotations back then. His arm just didn't need to recuperate after throwing that hard for the entire game day after day? That is HIGHLY unlikely, therefore it's reasonable to assume that he was not, in fact, throwing 100 mph in his games and his in game velocity was quite likely much lower, or his arm would have fallen off.
Could he have thrown 100mph outside of a game max effort? Sure, I can't say it's impossible, but I can make reason based judgements that the way pitching worked back then it was highly unlikely that he was doing so in games.
second, what did they actually do for training? Almost nothing in fact. There was no studying pitching mechanics, no weighted ball programs, no real understanding of kinesiology (which only began being truly studied in the 60's), these guys literally just showed up at the ball park and tried to hit the catchers mitt. Repetition was their only real training.