I have spent most of my day and part of yesterday so far looking for an explanation to that. I'm not an expert on memory and don't know what the limiting factors are for knowing the maximum capacity of memory a system can hold but my guess would be that the i7-965 wasn't tested by Intel to find out if it supported 8GB modules because they probably didn't exist (or at least exist for desktop computers) when the CPU was released.
Might be like how pretty much all CPUs can run RAM well beyond their rated memory frequency. The i7-965 extreme says it only supports up to 1066MHz DDR3 but many people have gotten much faster frequencies than that.
However, I did learn that the same 8GB Patriot memory module used in that review is going for $77.99 (free shipping) and with the mail in rebate it only costs $47.99, making it affordable to have 8GB modules right now.