Steinhaus was Polish, the confusion probably comes from the fact that he was born in then occupied by Austro-Hungary part of Poland (city of Jaslo, now within borders of Poland).
I've noticed more than a few Poles saddled with non-Polish roots in various history books. Banach himself was at one point incorrectly identified as a Russian mathematician in an old edition of the Encyclopedia Britannica. Sometimes the cause of error may be the historian's ignorance, sometimes nationalistic appropriation by others. Many Poles also often worked abroad during the 19th and 20th centuries because of foreign oppression by the Germans, Russians, and the Austrians (the last to a lesser degree than the first two).