@Anthony-Key As far as I know, it is not possible to remove the retroarch menu, what you can do is to configure in the mapping of the shortcut keys, some combination different from the standard to open the retroarch menu.
I think the tutorial you are referring to is this:
https://recalbox.gitbook.io/tutorials/gpio/connect-a-comparative-coin-mechanism-to-a-raspberry-pi-all-versions
