I realize the point of the article and many of the comments are about using homebrew ML/computer vision, but if you want something quick and easy, commercially available robot lawn mowers already exist. However, they tend to use boundary wires rather than any sort of vision system (some have gps, but I'm not sure if that's supplemental or can be used alone) though I think some have obstacle sensors for safety.

http://www.husqvarna.com/us/products/robotic-lawn-mowers/

http://www.deere.com/en_INT/products/equipment/robotic_mower...?

http://www.honda.co.uk/lawn-and-garden/products/miimo-2015/o...