Just because it may have been described as a "2 core PPC" doesn't mean that it has to be a 970MP. Also, as for the MR thread, are you certain that doesn't refer to using the Power Mac G5 as the XBox 2 development kit? The latter was certainly true, but the existing dual Power Mac G5 systems are far from being the same as a dual-core 970MP.
Frankly, there is no way the XBox 2 could use a full-blown, three-core 970MP design. The cost of that chip alone would radically exceed the price of the entire console. Power use and heat generation would also be a problem. As per the recently revealed IBM application note on the 970MP, it's going to be a pretty power hungry and hot chip. Contrast that with the Sony/IBM Cell design which is supposed to use as little power as the best of today's mobile chips. In that case, some have suggested that the single-core POWER derivative that is used in the Cell design might only require 10 to 15 watts and that the eight vector units would add only a few watts per unit. However, a single, dual-core 970MP is rumored to require 50 to 100 watts per processor even at a fairly low clock speed (circa 2 to 2.5GHz). However, the XBox 2 processor is rumored to run at greater than 3GHz.
Can you provide a link to the MR discussion that shows (or even suggests) that the XBox 2 prototypes used a 970MP?