As for stiffness, until it's tested we don't know what's stiff and what is not. For example, every Japanese rider I know claims that frames get softer as they get older/used. But all science indicates that frames generally (metallic) get stiffer as they get fatigued through work hardening. What about carbon? Carbon doesn't fatigue, but maybe the resins breakdown with flexing or age? Deeper rims are often considered stiffer than shallower rims. However, if you don't do it right, the longer rim walls end up as a point of flex such that you can actually get break rub on a road bike when you lean it over in a turn.
So the Mavic Io. It looks really stiff. But it's not triangulated like a spoked wheel. Is it stiffer? I don't know, and I wouldn't trust myself to make the judgement. I do believe that confidence in equipment is important to go fast, so I can see why so many sprinters might convince themselves that it's a stiff wheel.
However, we don't even know if stiffness is even important (within reason).
All I know is that I can go reasonably quickly on a 25 year old disc and deep front wheel and I'll consider the evidence as we see more data.