RF Via fences/stitching spacing
Spacing your vias by 1/20 wavelength is very unlikely to cause any problems.


  Does the frequency actually relates to the fencing/stitching space, or placing the vias closer than the smallest λ/20 (for fencing) and λ/8 (for stitching) is all that matters?


The frequency and the wavelength are intimately related, so the operating frequency has just as much to do with the via spacing as the wavelength does. 


  Is all that stitching shown in figure really necessary from an RF point of view?


Probably not. But for a low-volume board like a demo board, adding a few extra vias is cheap insurance against having to re-do the layout in the unlikely event there's a problem caused by spacing the vias too widely.