A mashup of what? Sounds all Beatles to me. But, I only listened once. It sounds cool no matter what it is.
What I meant was a computer with music editing software, and the two songs, one on each track. Or any one of a number of similar things. Then blend together according to taste.
If it
is a true Beatles product, that might be what John did - take home some takes of Revolution #1, and blend them with the rest of his and Yoko's ideas and call it #9.
Harder to do in those days, you needed multiple tape recorders, etc.