Poul Anderson mentions King Arthur in The Boat Of A Million Years, in The Merman's Children and in the Notes to The Dog And The Wolf (co-written by Karen Anderson) but Anderson's heroic fantasies and historical fictions do not include a King Arthur novel because Anderson was busy writing about many other characters, whether legendary or invented.
However, the Andersons indirectly link Arthur, King of Britain, to (their version of ) Grallon, King of Ys. In their King of Ys tetralogy, the Romans withdraw from Britain. The Romano-British Gratillonius, no longer King of the now inundated city of Ys but still a popular leader (Dux/Duke) in Brittany, expels the Romans and organizes local defense against barbarian incursions. Meanwhile, the British defense effort generates the Arthurian legend. Thus, the blurb on my The King Of Ys rightly says, "Before King Arthur, there was the King of Ys..."
Arthur and Robin prefigured two later social policies -
Arthur: the Round Table = equality.
Robin: robbing from the rich to give to the poor = redistribution of wealth.