IMHO it only comes down to the Skus and the wider range of possibilities they provide.
I would say it's fairly legit for those dev to claim their games, as they are, would not run on the 360... PS3 7 SKUs are capable of handling different tasks at the same time and without interfering with the main processor. The 360 has namely two processors (one divided in two) as powerful as these two processors might be they still have data processing limitations.
I'm gonna apply a Formula one analogy here : when a race car makes a pit stop you have a mechanic allocated to each task to perform, one who unscrews the bolts, one who changes the wheel, one who refills the tank and so on (that's the Cell). This method allows not only to be more efficient but also to be faster. Now say instead of these seven mechanics you have only two at your disposal. If you want to perform the same level of maintenance it sure gonna take more time and be less efficient because a mechanic, as strong and fast as he is, has only two hands so If you want to stay competitive then you have to make choices and give up some of the less vital tasks.
That's why games which take full advantage of the PS3 Skus cannot fully run on the 360. That doesn't mean a 360 version is impossible to do it's very much feasible but only by making compromises. At least that's what I think.
