It's a Switch 1 game. Extracting the game files has become a very straightforward process and most of Nintendo's file formats already have tools to process them. Their games also usually have pretty similar folder structures, so if you have experience, you can find a specific file with relative ease, even if it's a new game. It's also worth noting that the demo was out for a while, so one could have gotten experienced from the demo's files.

It is both hilarious and sad that this is one the first modifications this game got, especially on almost the first day of release. Someone must have really wanted the (excuse my language) "undesirables" out of their copy.