The thing is, they use their own franchises to make Easter Eggs of. The only times I can think of a Sonic reference in a Mario game was the blue hedgehog enemies in Super Mario World 2 back in the early 1990s, but that was a parody of Sonic, since Nintendo and SEGA were going through their console war.
Concerning this, god forbid Sonic is in the game. I'm calling shenanigans on this. Besides, you'd think that's be the first thing Nintendo would make note of for this game if it was true - they are the biggest stars in gaming, so if that wouldn't gather huge attention, nothing would.