Here's an explanation that actually might make sense, posted by Erik Lindeberg on cycleforums. In fact, if the solubility and diffusion rate are responsible, then we might expect that different tube compositions would affect the diffusion rate, and thus the widely disparate claims of CO2 leaking or not leaking could all be possible.

CO2 Inflators
> I've heard that
> since CO2 is a smaller molecule, tires lose air quicker.What has been
> people's experience with them? Viable alternative? Weight & reliability?
>
Small molecules e.g. helium, may diffuse faster than larger molecules
through some materials. The CO2 molecule is, however, larger than
the main constituents of air, oxygen and nitrogen, both with respect to
molecular weight and size. There are other properties,
though, that contribute to how fast the gas molecules
diffuses through the rubber, in particular how easy they
dissolve in rubber.