Well, yes (1) and no (2).
There is a difference between load-bearing ribs and non-load-bearing ribs.
(1) Yes, the 36-cell thing is purely marketing.
(2) No, the 21/27-cell thing is not purely marketing.
The diagonal ribs (=the crossbraces) are load-bearing ribs on both the Velocity, the VX and the Onyx. They pull on the top skin via the suspension lines. That's why they count as "cells". 21/27-cell canopies really have 21/27 cells. That's not just marketing hype.
On the other hand, the central vertical rib on the Stiletto and the Onyx is not load-bearing. Similarly, the two central vertical ribs on the Velocity and the VX are not load-bearing. Therefore it does not count as a "cell".