As a counter-point I've always heard them shortened to Mando and Canto (as in, "do you speak Mando, Canto, or both?") amongst Asian-American friends. Never thought anything of it really.

I've heard this a lot, especially when referring to both Mandarin and Cantonese, ie. Mando and canto

I've heard chinese pop music referred to as either c-pop or mando-pop

My Chinese friends do. Not sure if regional. I think it's just the logical shortening when the other option is 'canto', but it doesn't really make sense.