It seems that by pedantic UK parlance he would have been a folksong singer. Some of those from whom he learned his songs might have been folksingers. I'd never heard of him.

Brasser Copper (who technically undoubtedly was a folksinger) was born in 1845, so it seems very unlikely that even if Sandburg was a folksinger he was the first - indeed the essence of folk song makes it pretty well impossible, if songs like the Cutty Wren can be dated to the 14th century.