If you’ve try depressing the rear of that tab; as I remember, it lifts the front and the tab it captures is released. If you can’t get it apart, then I would try using carb or contact cleaner to clean out any substance acting like a binding agent between the plastic surfaces. Then, I would put a very small amount of silicone spray into the same location.
These are very old plastic parts that have become brittle. Patience is required, press the tab, Rock the connectors side to side, pull with constant pressure. Good luck; I know these things can drive you crazy,