I think a more apt analogy might be some other sport - there's too much difference between simple knowledge that requires minimal skill development (first aid), deep knowledge that requires extreme skill development to be used at all (brain surgery), and physical skills that can be learned and developed at a useful level with minimal knowledge though knowledge may help (basic martial arts).
So, let's look at training for basketball. You can learn to dribble and shoot entirely on your own. You could also learn the rules on your own. If you have a partner, you could even learn to shoot against a defender and maintain control from them when dribbling. However, you're only going to progress to a very basic level in most cases. If they have a deep flaw in their game, it will likely lead to a deep gap in your game in the respective area (if they are really bad at defending against a layup, you may develop a layup that any competent player can defend). And with just one person there, there're a lot of flaws you can get away with that just won't matter. Bring in a moderately good opponent, and you'd be in trouble. A coach joining your sessions would immediately be able to improve on your game.
And that's without even mentioning the inceased risk of injury in training martial arts improperly. For that, American football is probably a better analogy. In fact, I wish I'd started with that one.