I have a JD hydraulic auger built for skid loaders that appears to be several times bigger than your link. If the point on my auger happens to find a small, say 3" across rock, it will stop all progress immediately, even if it's only a 1/2" thick it will only spin on top of that little rock. If I'm lucky and wiggle things around I might "kick" it up and get going again, but it doesn't take much of a rock before I need a digging bar.
With a more commercial type auger, I could go out and buy a rock auger, probably for just under $1000. I'm doubtful that you'd be able to drop by a local heavy equipment dealer and find augers for that unit or that it could handle it, and also didn't notice any gear reduction either. The motor appears to be directly coupled to the drive connecting point.
Depending on your definition of "rocky" I wouldn't risk the $1000.
All of my comments included having down pressure.
I'd suggest renting a SS with an auger for a weekend and make your own decision based on your success. I hate popping a guy's bubble, but these are my experiences and spending probably a bit less than half of what that little auger would cost you for a days rent, hopefully you'll at least have a bunch of holes at the end of the day.
