Breaking in by playing would of course be better. For a retailer, though, it is more cost effective to use a mechanical device to break in new acoustics than to pay people to sit around playing guitars.
I didn't believe it myself either, until I played an acoustic before/after being broken in using a different device than this. It was more like an e-bow that went across all the strings.
After the instrument sat being vibrated over a weekend it sounded louder and better.
It all depends on how the instrument is built. Some acoustic instruments are "stiff" from the factory and need to loosen up as they're used for a while. Just like some speakers are "stiff" from the factory and need to break in.
Others sound more or less the same regardless of how much they're played.
The models that are built "stiff" sound better and sell better after break in.
All that being said, I still think doing it on a solid body, bolt on instrument won't do a damn thing.