How do corner vias in a QFN layout ensure stable operation?
I haven't seen that before, but on the previous page of the datasheet they show an example layout with ground pours coming "out" of those 4 vias.  In particular two pours are surrounding the RF in trace and two are surrounding the RF out trace.  I imagine the intention is to ensure that the ground surrounding the RF is the same ground as the IC's ground.  Looks like this part is meant to operate up to 20 GHz so your ground needs to be super stable/low noise/low impedance path.  So I think the goal is that you're directly connecting the pours surrounding the RF in to the GND pad of the IC rather than using vias from the IC to a plane and from the pour to the same plane.  Long story short I think they just want you to have a super low impedance return path, which makes a lot of sense especially in the RF world.