If there's anything all these people share, rather than study habits, that's a healthy dose of OTB play, and probably analysis with stronger players as an accelerator.
Oogie's way strikes me as the exception rather than the norm. All strong players I know spend more time playing and analyzing than studying books. I don't say it's detrimental, but chess being an 'applied science' game, practice certainly makes perfect... sense.
You are right, all of the strong players suggest regular OTB play at longer time controls is a required component. What is the reason? OTB play must be special, in that it accomplishes something that can't be easily reproduced with other activities. What does it accomplish that can't be done alone?
In rated OTB long games, you have to use a specific set of skills which you usually don't train in isolation such as : focusing for many hours and deal with fatigue / concentration lapses, taking practical decisions vs. looking for the optimal moves, choosing when and how far to calculate, dealing with emotional blows (ie. unexpected moves, mistakes), time management, risk taking, etc. It's a bit like marathon running : your body needs the experience to learn how to best regulate itself on long distances.
Besides, because of the greater emotional involvment in OTB play, the feedback loop is much stronger (desire to learn after a loss -never again !- and long term memory triggered by emotional markers).
Different people learn differently however. Playing games (especially practice games) is beneficial but application in the field isn't the same as doing the prepwork. Lawyers and scientists need to go through tons of college level textbooks to get into their fields and likewise chess has its special areas that need to be learned. Granted the stakes are much lower in a game (losing in chess for example doesn't allow a killer to get away with murder or an innocent person going to prison unlike law) but still.
The Russian school of chess greatly emphasized a rigorous study and training of the game, especially endgames. I Trust Botvinnik's observations (and Capablanca recommended serious endgame study before him!) on chess improvement.
As the poster above me stated, the goal of the Russian school was to produce GMs. And once again, they did not have the technology or transportation etc that we do.
I agree that people learn differently. That is why I post a counter-argument everytime you recommend someone to read 100 books in order to improve their chess.