Okay, so you've generated seven addresses whose public keys once summed in secp256k1 give you a public key whose address has bitcoins. So what? I can do this too and form a fancy saying using dozen of addresses whose public keys once summed in secp256k1 will give you satoshi's public key.
Can you sign a message with that public key?
In the OP example there is a valid signature. That means someone has (or had) the private key for the public key.