The --expiry parameter to bin/php/ezcache.php will only have an effect when used together with --purge. This is by design. This doc needs to be amended:
https://doc.ez.no/eZ-Publish/Technical-manual/4.x/Reference/Scripts/Generic/ezcache.php

The entry for --expiry says:
Expire caches that are older than the specified date. The date must be provided according to the GNU Date Input Formats syntax. When combined with the "--purge" parameter, the script will clear the caches instead of expiring them.

This could be changed to:
Expire caches that are older than the specified date. The date must be provided according to the GNU Date Input Formats syntax. This can only be used together with the "--purge" parameter, which means the script will delete the caches.

Example 7 must also be amended. It says:
php bin/php/ezcache.php --expiry='-2 days'
Expire all caches that are older than two days.

This could be changed to:
php bin/php/ezcache.php --expiry='-2 days' --purge
Clear all caches that are older than two days.

Additionally, the doc within ezcache.php itself should be amended accordingly.