Bitcoin message sign isn't about message encryption/decryption, but make sure anyone can verify that it's created/signed by whoever own private key to a Bitcoin address and prevent message tampering.
Theoretically, signing something basically is encrypting and decrypting (in reversed order).
Usually, to encrypt something, you are using the
public key of the recipient.
The recipient then can decrypt it using his
private key.
When signing, you are using your
private key. And everyone else can use your
public key to verify it.
In fact, both of these (encrypting/decrypting and signing/verifying) are using the same concept (just with the private- and public key being 'swapped')
But.. in the end, you are right. Signing and verifying is NOT encrypting and decrypting (at least how OP was illustrating it). It is the same concept.
