Barring a pre-crash sequence mechanical failure, this was pilot error. There is a safe speed called Va, or maneuvering speed. While most aircraft will have higher maximum allowed speeds, maneuvering speed is the most important figure for aerobatics because it's the maximum speed at which the airplane's flight controls can be used at full deflection (i.e. maximum effort) without exceeding the structural capabilities of the aircraft.

What most likely happened was was a loss of situational awareness in the pre-loop dive, resulting in a steeper and faster dive than intended.

When doing loops there are rapid changes of acceleration, specially on inverted loops. It can make you pass out easily, even if you are young. Being 91 years old, I would be extra careful.

Over the wings in which direction, though?

Not a stall but it might've been loss of control due to aerodynamic forces and effects. There's two that come to mind when talking about leaving a loop too fast like what seems to be the case here.

There's control lockup, where the aircraft is flying so fast that the mass flow of air over the control surfaces is so high that the controls "lock in place" or "get stiff", meaning the force you can apply to your rudder stick in the cockpit isn't enough to overcome the forces acting upon the control surfaces.

Then there's control compression, where the airflow around the wing is so fast that the local airflow around the airfoil becomes supersonic (aircraft as a whole is still subsonic) and detaches after the point of maximum airfoil thickness, leaving no laminar flos for the control surfaces to work with. This will feel different in the cockpit, instead of the controls becoming stiff they get loose but no control authority is achieved because your control surfaces are in fully turbulent flow.

Both of these phenomenon were a big issue in WW2 during dive attacks, since enocuntering or even going beyond the max aircraft speed was common during dive attacks, you wouldn't be able to pull out of the dive and go straight into the ground at very high speed. Either of these could've been an issue for Anders.

You can stall a wing at any airspeed. It’s about the angle of attack of the wing exceeding the critical angle, not the airspeed. In accelerated flight (like the bottom of a loop), the airplane is only capable of so much acceleration/pitch rate before stalling the wing.

I agree that it was not in stall, based on reviewing the video. At that level of G a stalled wing would enter what's called an accelerated stall, which will almost always cause a rapid snap roll to one side or the other, as one wing will stall slightly before the other.