When I was a student in about 1972 a friend of mine decided he was going to get heavily into ragtime as it was a form of music that was ripe for revival. Infected with his enthusiasm, I tried to learn Maple leaf Rag and a few others from recordings made from pianola rolls... Only months later all those Joshua Rifkin recordings came out, and shortly after that, and I would guess not by chance, The New York Library printed all of Scott Joplin's works in a big book (all except one piece for which they had a copyright problem, I believe) I still have my copy of that book.
Don't forget that the movie The Sting also came out around then, and featured Joplin's The Entertainer as theme music.

Yeah, you're right - it could well be that somebody like Van Ronk said something to Rifkin that dropped the hint there might be a market for those recordings.