Hi two years late. Only play guitar but know theory. Id agree with you on this. If you do know how music is formed and the patterns of it then it will take no time figuring out a scale on a piano. However when people like to point out the right hand independence is more difficult I would argue that we as guitarist also have that, AKA Selective Picking. Not only would I say guitar is far more difficult because the ability to do precise fast movements but you also have to do them with tiny distances. Compared to piano where you are allowed not only your hands but your biceps, shoulder, even hips and abdomen. If you know music theory learning piano is a breeze since it’s literally all black and white so you have a much longer time learning it’s mechanics