Photoperiodism: Short-Day Plants
I think you are looking at other the wrong way around. There is no critical light period, there is a critical dark period. That long-day flower is also a short-night flower. As long as there is never any period of darkness that exceeds the critical dark period, it will flower.

In that second picture at the bottom, exactly that is the case. The dark period is not long enough to prevent the flowering. 

And in the bottom example of the first picture, there is no uninterrupted dark period longer than the critical dark period, so the plant never flowers.