I played this twenty-eight or thirty years ago, and it was one of my first "findings" of the real, traditional American music (I assume the above is talking about the middle section, the gospel part in Bb), in addition to transcribing Janice Scroggins's introduction to "The Maple Leaf Rag" at about the same time.
My piano teacher was not especially happy about me asking her to write, what she called "pop piano" chords above that section (you know, like Bb6, F7+/A and so forth), but she seemed to enjoy teaching me that piece. Of all the things I played on the family upright, that's the only time I could feel my father's annoyance from the next room, while he was watching the news.
I was just reading through it a month or two ago and the other one of that set I did at the time, "Down by the Riverside," and, yes, playing that gospel section faithfully is quite a bit harder than I remember it. "DBtheRiverside" is harder, and I don't think I ever polished that one up to the end. D maj. is a key you have to learn by experience to love for American music. I did, however, do all of "Winsboro," although I didn't memorize it, and have the pencil marks to prove it! Funny, looking back at those all those years later.
It's easier to play Ray Charles "Sweet Sixteen Bars," from the record, to speak of tunes in Bb, than to literally play the Cotton Mill blues.
I also noticed that it doesn't sound that great on a digital -- at least with my speakers/amps. Needs an in-tune acoustic to really make the effect of the driving mill come through.
How difficult? I don't know. I was eleven or twelve, and I wasn't a prodigy. It takes some effort.