You are conflating music theory with musical understanding. These two things are separate. For example, many singers know nothing about musical theory, but if you ask them to sing a song in a different key they will do it flawlessly (if they know how to sing in tune). A guitar player can do the same, even because of the shape of the instrument allows this. Maybe this is harder for an instrument like the piano, but it is not a limitation of the player, it is the instrument that requires you to learn about flats and sharps.