I'm as skeptical of marketing claims as the next person, but it seems to me a frame using less material is going to be lighter, whether or not this translates to the complete bike.
Also, why the blanket statement that they're not stiffer, even if only to a tiny imperceptible degree that only marketing departments care about? How do you know they're not? Common sense would seem to suggest that the smaller triangle of a compact frame is going to be stiffer than the larger traingle of a traditional one - again, even if this is offset by the need for a longer seatpost (and when riding out of the saddle, seatpost flex isn't an issue)?