Well, their claim is not a HOAX, although it may not be entirely accurate in practice. We need to distinguish between 'air' mats, mats with insulation inside, and the NeoAir mats. To explain:
Blowing up a mat with your mouth puts water vapour inside the mat. We know that. What happens to the water depends to some degree on what sort of mat it is.
If the mat is full of internal insulation - eg a DAM, then the insulation can get damp and lose some of its insulation properties. However, see the next bit before drawing any conclusions here.
In the case of an 'air' mat, it is likely that the few grams of water will collect at the bottom of the mat and in really cold weather it will freeze there. The water won't collect at the top where it is warmed by you. In effect, it's a bit like a old refrigerator where all the ice collects on the surface of the freezer rather than all through the rest of the chamber: it goes to where it's coldest.
Now a thin layer of ice on the inside surface of the bottom layer of fabric in an 'air' mat is not very different from a thin layer of ice on the outside of the bottom fabric surface. We may be talking about a small area of ice only 1 mm thick perhaps, on the fabric. That is not going to have much effect on the overall performance: all the rest of the air space is still there. So there won't be a big change in the R value. That is, unless you really thrash around in the night and stir the air up - which is where all 'air' mats fail in the cold.
Coming back to the DAM, the same thing applies: the ice will mainly collect at the bottom surface. In this case, a bit of thrashing around won't have as much effect. So the loss in the R value of the insulation should not be too high in practice.
Where does the NeoAir fit in all this? Well, it is really just a complex multi-layer 'air' mat, so the water should still collect on the ground surface (ie at the bottom). Provided you drain the mat out when you get home, there should be no long-term effects on the obvious insulation value. So far, so good.
But the aluminised coating on the internal baffles is another matter. In theory the water should have no effect on it, but field experience suggests that the coating on the early models was not good. It rubbed off. People experienced this. Mind you, I am not sure whether the aluminised layer really did much for the R-value anyhow. Regardless, I have to say that water inside the NeoAir mats did seem to cause some degradation on the early models. I don't know about the more recent models after CD tackled that problem. I would expect that CD would have largely solved it: they are fairly competent.
Anyhow, I think CD was willing to replace mats where there was a major problem - they are good that way.