Should we mention how Splatoon files were found in Mario Maker?
AKA, it doesn't mean anything. Don't get your hopes up.
Splatoon was out before mario maker... Which means the situation is completely different. Not to mention the fact that
they're two completely different goddamn companies. This is valve purposely putting in hints at the game that they know almost all valve fans want. There's literally no reason for it not to be in development, they've given way too many obviously intentional hints at it.