The feds removed the fingers of the perp and used his fingerprint to enter the phone. This process requires an undertaker that is on the take, not much else.
First, the iPhone fingerprint system will not work with fingers that are dead. It requires living fingers. So sorry, your theory fails on that problem alone.
Second, the iPhone 5C the San Bernardino Public Health Department gave to its employees has no fingerprint sensor. Ergo, a finger, dead or alive will not unlock it. So sorry again, it won't unlock by a non-existent TouchID that doesn't work on an iPhone which doesn't have one.
Nice try.