I received DMCA from Nintendo in 2015 from dmca@millernash.com which was also confirmed to be legitimate as authorized agents.

Big companies like Nintendo doesn’t have to use their own in house corporate counsel for this kind of enforcement. They can and often do task it out to firms that’ll take on both discovery and take down based on given directive on an agreed rate that’d be cheaper than them doing it in house, so they don’t need to train up an entire department on the skill set required.