You can't post the encrypted messages before the "reveal date" because the recipients will be able to decrypt it right away.
The idea is not to let the recipients know about using their public key until OP wants to call them to check it was signed for them.
But what's the difference in terms of security?
Both signing a bitcoin address and signing using pgp key are secure enough. PGP actually used in military system. The regular email system also use PGP in the back to encrypt the message. From the font we can not see it though.