From the website of GNU Privacy Guard (GnuPG):

If an attacker could decrypt a session key it would only be useful for reading the one message encrypted with that session key. The attacker would have to start over and decrypt another session key in order to read any other message.

From my understanding, if an attacker is able to decrypt a session key which was encrypted through a public key, then the attacker practically has the corresponding private key pair.

Hence, it is not difficult to decrypt any further session keys encrypted by the public key.

How does a session key make this any more secure? I can only see this as redundant security.