It was an interesting paper. To summarize, they propose the propagation mode when foF2 is too low is via ground side scatter. "In the ground side-scatter mode, the transmitted signals are scattered from the ground at a range exceeding the skip distance, and thence to the receiver. "
We do receive stations (on 43 meter for example) when it should not be possible via normal propagation, based on the real time foF2 values, so this could be one way we're hearing them. Another mechanism is sporadic E, of course.