You’re correct, you can pirate melee to play on slipping. However, Nintendo has absolutely 0 proof of any of the players that will be taking part in the tournament doing so. The vast majority of people involved in melee have access to a melee disk. The game has existed for 20 years IRL, remember?
Decades of playing melee on disks and GameCube, christ’s sake. They own the game and can simply use their own copy. They have absolutely no grounds to call it piracy, and they are unambiguously lying by saying it requires piracy. It doesn’t “technically” not require piracy. It just doesn’t. Period.