Thats right. Banks/Funds/Institutions still have overnight departments to manage their positions.
Dollars aren't owned by the US Government and therefore cannot impose trading "holidays" on the dollar.
US Institutions may not operate on a Statutory holiday (Even though many trading departments will still be open), yet dollar holders around the world (UK, Asia etc), can still buy and sell their dollars freely without US participation. Hence the FX market will still be "open".