I already mentioned this in HN discussion about the same subject, but that ball doesn't track what it has cleared. It doesn't really matter how much mines you blow. It matters how much mine free soil your cleaning effort creates. And it has to be 100% clean and clearly marked to be of any value. You could track what it has cleared with some kind of GPS signaling device. But being a ball, it doesn't clear that much on one go so you have to have quite precise signal. And it might jump over some mine. Or just roll over some mine that is malfunctioning little bit, but might explode under a truck. But I think this is a good publicity stunt as it might get designers to develop mine clearing hardware with a mindset: "this doesn't have to be expensive".
According to the designer, he envisions this ball as a good way to find minefields. Apparently, soldiers can spend days looking for mines in a place where there may not be any. He think that you could let this device loose and let it look for the fields. Then when one mine is found, then you could bring in the mine sweeping teams. He doesn't claim that it will do a comprehensive sweep.