Gonna have to give that a try because it sounds like a very bad interpretation of the expiry mechanism in keys - encryption functionally never expires if you have the key, because you have the key. It's solely an error on the part of the sender to use an expired key, because it might be no longer available.

(admittedly AFAIK encryption is handled by key expiry poorly overall in GPG - the lack of perfect forward secrecy means an expired key which leaks can still decrypt all the old messages if they were intercepted).