That doesn't make it impossible to MITM - Apple still controls the keyserver.
When I ask for nardi's public key, they can give me theirs, I encrypt it with that key and send it. They use their private key to decrypt it, store it, and then encrypt it with your actual public key and forward it along, neither of us any the wiser.
When I ask for nardi's public key, they can give me theirs, I encrypt it with that key and send it. They use their private key to decrypt it, store it, and then encrypt it with your actual public key and forward it along, neither of us any the wiser.