If you lay your hands flat with each thumb pointed at the other hand, your left hand will form an “L” shape. That’s probably what they meant to say.
Edit: oh looooooool, I literally just didn’t get the joke
This actually makes sense. I always assumed people know which one their dominant writing hand is and go from there. That’s what i always did until i just knew. Like when someone asked me, i made a fist or something with the hand i was writing with