CAUTION: wear safety goggles to watch this video.
It may seem nuts but this is a similar blade setup to the Husqvarna Automowers ie. multiple swiveling 'razor' blades around a large spinning disc. I'm just running a little smaller at the moment.
When the drill had a charge (!) it was very effective at nibbling the grass down. It's easy to see how this can work very well as the 'bot passes by repeatedly.
What do you reckon the chances are of the robot being able to identify weeds then stop over the top of them and say: zap them with a laser beam! ? — that's one for the boys ;)