Well, there is some misunderstanding.
Rulebook, at p. 29, 26.1 says:
It can be agreed between players that the above additional cards (those which don’t have an elephant icon in the left bottom corner) are ‘Remove After Play’ cards and get removed when played for an Event.
There are some Strategy Cards in basic deck (1-64) without Elephant icon, but they can be used in Hannibal or Hamilcar scenarios.
Optional rule is only about new cards 65-91.