Called a 'friction lock' -- just grab and pull.
Yep, Friction lock. The locking tabs on the receptacle (top, left connector) don't move, the top of the slot on the plug (bottom, right connector) flexes outward as you push connectors together or pull them apart. Tell tale sign is the shape of locking tabs. It's triangular so it slides off as well as slide on.
The thing that holds it together so tightly is the fact that they have been mated together for many years. Do as Chas said -- wiggle them apart.
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