Not exactly. He only controls the codebase for the publicly acknowledged Ethereum. He doesn’t control every private key, or the data itself. Anyone can make their own “Ethereum” but everyone agrees this is the correct one, so those others are just imitators. That’s why there are so many altcoins. All the computers that want to run eth around the world have a copy, and all the copies have to match otherwise everyone knows they are fake. You could take Vitalys codebase today, make a few tweaks, and publish your own shitcoin if you wanted. Many people do. Almost all coins are just copies of either bitcoin or ethereum codebases.