I know the definition and it's not relevant to this conversation. Using your argument would mean a single residence with multiple kitchens is not a single residence, IOWs a single residence can only have one kitchen.Here's the definition from the NEC which controls the use of (2020) 310.12:
"Dwelling Unit. A single unit, providing complete and independent living facilities for one or more persons, including permanent provisions for living, sleeping, cooking, and sanitation."
Would you explain how it's possible to interpret an ADU as other than a second dwelling unit? I'm not seeing it.