I think having command means: pitcher is hitting his spots, generating a lot of K's, ground outs, fly outs. Command of spots, Command of the game.
I would call "spots" as areas he's trying to get the ball to, not like throwing darts. Throwing dart like would be too precise (I think), promoting to guide the pitch and not trust your release. If you're trying to throw a high fast ball to get the batter to swing and you end up throwing it right over the plate, you just missed your spot. The batter likely punished you for missing your spot. The pitcher is lacking command.
Developing and practicing good mechanics puts you in strong position to repeat good mechanics under pressure - game time. You have a chance to hit your spots to produce outs.
BUT - like any competition: the mental game. You can have the best and very repeatable mechanics, but if you're not dialed in to the immediate part of the game, able to avoid distraction, you're not going to have much Command, you won't be on the mound long.
Great athletes have the mental ability to get in and stay in the "zone". The mental zone allowing your great mechanics, athleticism, game IQ to work when you need it to. Any great athlete, great trader....