To be clear, they have test data from precisely 1 game - Portal. It's also a smaller sample size by far than their Windows install base. Make of that what you will.
Assuming it's true, which I don't necessarily dispute, it's not really shocking considering the tight control Apple exercises over its hardware. A ton of the Windows side machines that are being compared to are almost certainly either much older, or much lower spec/cost in hardware. The fairest comparison would be like-for-like hardware *and* cost. So let's say a $500 budget Winodws machine with cut rate hardware is comparable (Core 2 Duo or whatever) to a low-end iMac at $1000 or something. Still not a fair comparison because the iMac uses better brand hardware, with better drivers, etc. Let's compare a $1000 PC to a $1000 Mac and then we'll really see what the stability comparison is on the same *class* of system. That's the real key... Basically Mac comes out smelling like roses in this comparison because they don't *have* machines in the class of system that are likely causing most crashes.