100% with kharmael, the basic lifting force of the wing is based on the reduced pressure above due to the increased velocity, but also slight increase in pressure on the lower surface.
However as the critical angle is reached the sparation point on the upper surface moves forward and the reduction of pressure on the top surface decreases.
The flat plate theory, is based upon the idea that lift is created in a combination of low pressure above, (now gone), but also the high pressure below, hence as the AoA increases the high pressure below the wing plays a large role, and Cl is able to see slight improvement.
Paper aeroplane theory! No real aerofoil but yet can glide very well!
My 10c anyhow.