All right, for those having trouble, you need to, before starting the game, go to the home screen and ensure that the switch is registering your disconnected joycons as two separate controllers.



If it isn't, go to your "controllers" option along the bottom of the Switch home menu and set it up similar to what is here: https://twitter.com/flameflash/status/917961680310677504



You may need to choose to "change grip/controller" and hit the top ZR/ZL buttons on both undocked joycons but then on the home screen again it'll show a single joycon, horizontal, which means you can start the game.



Default settings are then to hit ZL twice for two credits and then ZR once on the 2nd player's joycon and you finally have your Luigi player.



All credit for the fix goes to the hamster.co.jp support team who were super quick to respond to my email answering the question.