Maybe it's a regional thing within Poland. I have no Polish roots against which to measure the fact that some of the Polish singers I've shown it to are familiar with it (and as you say, it's well documented on the web) while others, despite being proficient folk singers with large repertoires of kolendy, have not heard of it (they tend to say the tune is familiar but don't know words). I got it from the International Book of Christmas Carols (Ehret and Evans) years ago.