I think you are probably right - it played out OK without discarding down and as you say the rules don't mention ever discarding down. I was just going through a moment of uncertainty!
Played it the same way tonight. There is no mention anywhere in the rules that you must discard event cards to your intelligence level. Only fill your hand to your intelligence at the end of your turn (or before if you are that one race). If you happen to have more Event cards than your current intelligence, more power to your aliens!