I doubt that the very slight added weight would significantly inhibit vibration of the top of a giutar; in the video, the device looks to be made of VERY thin flexible plastic. I also doubt that a valid judgement on the gizmo's effect upon sound/tone could be made by a comparison of with-and-without sound bites that have been (a) recorded by who-knows-what technology and (b) played back through the internet.

I'm somewhat intrigued, and if I could afford thirty bucks as readily today as I could have a couple of years ago, I'd give one a try.
Money's a little tighter now; I can live with the current sound of my very nice old acoustic-only guitar as-is, thank you. But louder and boomier would be better, all else remaining equal...