You heard wrong. Straight from FI "Working well in both small ported and sealed enclosures." Ported boxes will hit every high and low perfectly as well if built right - while being louder. Tune to 30 for the 18 and you can hit down to 22-24 with ease - and that is well below what music will ever play to.
I know what the site says...even though it says best for both ported and sealed, its best played in sealed box from people opinion. Have you even heard the sub played in a sealed or ported? its best played in a sealed.