Two buttons, and neither of them aims
Look at the control scheme and the whole design falls out of it. Each player gets exactly two keys - one to jump, one to shoot - and that is the entire input. There is no walking, no crouching, no cursor, no aiming of any kind. You cannot point the gun. You press shoot and the shot goes wherever your character happens to be holding it.
That sounds like it should make the game shallow, and it does the opposite. Because you cannot correct your aim, the only variable you control is where your body is when you pull the trigger. Every jump is therefore an aiming input as much as a dodge, which is a genuinely unusual thing for a shooting game to do. Players who treat jump purely as evasion tend to lose to players who treat it as the setup for the next shot.
It also means nobody is at a disadvantage for not knowing the controls. Two keys is a five-second explanation, which is most of why this game works so well when you hand it to someone who does not play games.