The basic structure designed in the 1960's, when precision weight measurements with onboard sensors were not around, the plane was probably designed for MTOW to be [worst case scenario + 15% or more]. The engineer of "yesteryear" would not place a absolute maximum anywhere near the actual lift limit of the wings, or structural limit of the airframe. Carrying more to even more weight than published should not significantly endanger the flight.
The pilots forgetting to configure the wings for takeoff is a rare event, and having the takeoff configuration warnings disabled for whatever reason is a rare event as well. Having both happen on the same flight is extremely rare and two cheese holes that have to be almost forced to line up (seems highly unlikely).