"When the Chariot Comes" came into print in 1899 according to the Traditional Song Index (cufresno); one authority quoted there (Fuld)says he knows of no printing of "She'll be Coming Round the Mountain" before Sandburg in 1927. The song was sung by children in my primary school in the 1930s and certainly was known to my grandparents, who probably never heard of Sandburg.
Joe Offer's speculation is one I can agree with, until more evidence is found.