There's a rather large key index on the start screen, although you're not the first to mention having overlooked it. Those are the official glyphs for pico-8, which supports 2 player controls. So each player has a o/x button and a different set of directional buttons if they're using the keyboard. We opted to stay true to pico, and compromised with the instruction display and a note on the game page. Might have to do some research and see how other pico-8 devs dealt with publishing to non-pico user markets I guess. Just for clarification, did you overlook the key index or just misinterpret it?
I missed it probably from playing on a 4k wide monitor and only looking at one side of the screen missing the ellipsis connecting the two. Since most of us are trained to read left to right, perhaps swapping their placement would help. It is there and very obvious once you mention it but some how it was easy for me to miss :/