Seems like Nintendo has had a lot of games leak early. You'd think they would figure out how they're getting out and then fix it... whether it's an employee leaking them, or just really bad security policies, it can't be THAT hard to fix the issue, right?
It's probably not one employee, but a few dozen that would be able and willing to leak games this early. Anyone high enough up in the ladder of distribution to have access to the games that early could do it, and there are too many possible points of egress for Nintendo to easily find the culprit(s).
What I do know is. Individual stores typically don't get games until a few days ahead of release (giving some margin of room for delays and shipping items out to customers), but warehouses belonging to chains etc. can get them weeks in advance as they need time to ship them out to all the individual stores. Anywhere between the warehouse and your local store, a game could just "fall off the truck".
And there are a number of people who would have had access to the warehouse and could've taken a copy during the time it's there, and even silently put it back after dumping if they were extra ballsy or just wanted to avoid it being discovered as quickly.
Even if the owner of the warehouse knew who did it, they might not report it back to Nintendo if they're in on it or they're bribed.
And if the warehouse owner isn't being helpful there's not much Nintendo can do, unless they don't mind cutting off the supply to that warehouse entirely, which could potentially cut off access to their games to every single store in a chain, which could mean a lot of profit lost for Nintendo.