I think he means that asteroids wouldn't remain after the conditions required to make new elements in the classic sense (adding a higher number to the periodic table). That said, I do think there is a nonzero chance that their could be new isotopes of elements already discovered embedded in some asteroids, but that's like trying to find a needle in a galaxy.

To answer your question and give my input, all of the elements we have discovered thus far have been either forged in the stellar furnaces or was brought to us by asteroids, comets and possibly solar winds. The remaining ones that we have observed for a tiny, almost infinitesimal fraction of a second were discovered by us. It would be irrational to think that all of the elements we've ever encountered were only here on Earth. Our planet has a rich history of interstellar visitors, and we have observed in our lifetime a meteorite with vastly different material composition to what you would find on Earth (Iridium comes to mind).

The feasibility of asteroid mining is what Planetary resources could be a ming for, simply a proof of concept for further missions to nearby asteroids. I suspect that they won't shoot for the asteroid belt first, but try to find one that's already scheduled for a close fly-by of the Earth. You'd use less fuel, less time, and less energy overall to reach your target if you can get a launch window lined up with the timing you need. I think they'll claim the belt first though in order to drum up interest and controversy (like what's happening in this thread), and hopefully turn that attention into money.

I'm excited to see their progress even if it turns out they fail. A net gain of knowledge about the possible avenues to asteroid mining in the near future will mean a good investment overall, even though the initial investment might fall at first.