To be fair, there are legitimate business uses for Mac. Video editing, for example. But I get your point. And I suppose a Macbook AIr wouldn't be what you get for that, anyway.
Delete the memory card file for that game, assuming you didn't change the default Nintendont setting which uses a separate virtual memcard for each game.
Ask the original seller if they have a clean NAND backup. Without it, you can't make the sysNAND 100% clean again. Factory reset does nothing. You can clear the logs with homebrew, but Nintendo can detect that the logs don't match what's on the servers. But it's better than nothing.
btw i feel this a good time to remind you guys that sea of thieves was made by rare, who also made DKC1-3 and DK64, alongside banjo kazooie and conker's bad fur day