It seems stupid to me.
First, it must feel a lot different than a regular stone because whatever you fill it with will move freely inside. This is the biggest downside that I see.
Second, it's $500, and you only get one size stone. If you buy stone molds, you could get 4 different sizes, and make each size in several weights by using foam or lead cores. Concrete is cheap. And then you can set them up so you use multiple sizes in one training session. Plus, if you know someone who has a mold, you could then make a stone for under $20. Or if you and a friend or a group of people who train together want to buy a mold, you could all have your own stones for minimal cost.
And third, it won't withstand drops to concrete. Though usually you don't drop stones directly onto concrete. The stone may hold up, but your concrete will get fucked. Even with mats and plywood I've seen pictures of seriously fucked up garage floors.