Actually you would address a letter to “Flushing” if writing to someone in Kew Garden Hills, and he definitely wasn’t matir an eiruv for all of Flushing.
From their website, it seems like it has to do with the highways. This being said, I heard they connected the KGH and Forest Hills eiruvin in recent years, which would mean crossing over the Van Wyck. That expansion cannot claim to have the backing of Reb Moshe.