What do they mean by documented proof? A legal opinion?

The Traditional Ballad Index has the date 1909 at the top of their write-up, copied by Richie, above, or go to the website: Search and enter "Nine Hundred" in the blank.
Also look in the Lomax reference, FSUSA listed on in the write-up.

Perrow has a fragment, "Fo' Hundud Miles From Home," in part 2 of his "Songs and Rhymes from the South," Songs connected with the railroad, No. 12, collected 1909 in South Carolina, and published in 1915.

See my reference to "Mountain Dew" in Brown, above, coll. 1914 North Carolina. I think Joe Offer can provide the full text.

All of these are before 1923, and NO author is cited.