Why would they release peak CK-MB greater than or equal to 100ng/ml
I fully believe it is a component of the MI portion of the endpoint. But MCU very carefully steers away from actually saying it is THE definition of MI. For an example, see the Alexion trials where they had an SPA (i.e. the FDA almost certainly put in the endpoint they wanted) and the definition of MI was not limited to CKMB>100.
Also note that Alexion had results in CKMB that were just as encouraging as MCUs and utterly bombed in the pivotal trials - my guess is that it is possible to change the way that the damage to the heart expresses itself without actually decreasing the damage. Hence the decoupling of CKMB from actual MIs.
Note again that the physician diagnosed MI rate in the last trial is the offsetting factor to this. But I'd want to see a published paper/poster expressing the fact that this was done blinded etc.