It wasn’t a DMCA. They advised the dude to take it down due to Nintendo bring Nintendo.
This information is really easy to look up.
People here are constantly claiming that Nintendo demanded from Valve to take action in order to not get sued. I suggest you complain to the paid authors of PC Gamer because they are the ones writing about Valve lawyers: “there’s at least a possibility Valve could end up sucked into a dispute over it. Even if it’s only tangential involvement, Valve’s lawyers may have just decided that it’s not worth the headache.”