Another angle: maybe "Macky Macky" is a term used in a specific region or community for a type of piano cover or performance. Alternatively, "Macky" could be a mishearing of another name. For example, maybe someone named Mackie plays the piano and gained popularity in a certain way. I should verify if there's any official information. If not, I can define it as a modern trend or internet meme related to piano covers. Alternatively, perhaps Macky Macky Piano is a specific piece of music played on the piano.
