PMP41031: Negative voltage clamping diode in VS pin

Tool/software:

Hi experts,

I am working on a Two-Switch Flyback converter based on this reference design. 

My question is regarding this part of the control circuitry.

If I am not wrong, diodes D18 and D19 avoid VS pin voltage to go bellow -9V, but the absolute maximum rating is -0.75V for this pin according to the datasheet. 

I think I might be failing to understand the pourpose of this circuit. Any help to understand this would be very useful.

Bests,

Raúl