Reddit mentions of Coco

Are you talking about the fictional origins of these lyrics? Because these ARE the original lyrics; "Everyone knows Juanita" is an original song written by Adrian Molina and Germaine Franco for the 2017 Pixar movie, Coco.
https://acadianasthriftymom.com/2017/11/29/the-meaning-and-love-behind-the-music-of-disneypixar-coco/ (the writing of Juanita is discussed 2/3 of the way down)
https://www.amazon.com/dp/B073LZD7PH/ (see back of CD cover, left lower corner for writing credits)

I too started researching the same thing you all are researching (what were the lyrics to the original Mexican song?), but then it occurred to me, what if it SOUNDS like an old Mexican song but it's an original song written for the movie? Turns out they did a convincing job of making it sound like a Mexican ranchera.

I do agree with the theory that the authors probably implied "knockers" with the euphemistic "knuckles."