I suspect that the report is kind of misleading in claiming the expired certificate as the problem. A certificate can not be used to decrypt traffic at all but rather the private key matching the certificate is needed. This is at least true for the (obsolete) RSA key exchange which I assume was used here - for inspection with DH key exchange one would need instead need the pre-master secret of each TLS connection and neither certificate nor private key would be usable.
Thus the problem is likely not the expired certificate but instead that the private key installed on the device matched the public key in the expired certificate but not the one from the new certificate. This is likely because the new certificate did not reuse the key from the old certificate but instead a new key pair was created. For successful inspection the new private key matching the new public key in the new certificate had to be installed on the inspection device.