The responder is initialized with a pre-shared long-term static key, which is assumed to be pre-authenticated out of band by the initiator.
Message A, sent by the initiator, benefits from receiver authentication but is vulnerable to Key Compromise Impersonation. If the responder's long-term private key has been compromised, this authentication can be forged. However, if the initiator carries out a separate session with a separate, compromised responder, this other session can be used to forge the authentication of this message with this session's responder. Message contents benefit from message secrecy and some forward secrecy: the compromise of the responder's long-term private keys, even at a later date, will lead to message contents being decrypted by the attacker. 1,2