The difference between the hobo symbols and the URR quilts is, "source documents". From the 1930's on, at the same time they were making the symbols, there was written documentation about what they were doing.
The URR - people wrote books about it after the Civil War. NO ONE mentioned quilt symbols. It was NEVER written down until recently.
This is how historians work - they go back and look at documents of the time they are studying - books, letters, diaries, etc.