My understanding is that the FBI is not asking Apple to decrypt the phone. The FBI is asking Apple to produce a version of iOS that does not lock out or wipe the phone when an invalid passcode is used more than 10 times. Then they want to try to guess the passcode using brute force methods.
This is the coding equivalent to commenting out an "if" statement, to oversimplify it.
McAfee simplified it too far, especially his claiming the user passcode is somehow stored on the iPhone when it clearly is not. If he knew anything at all about it, he'd know that lack of storing the passcode is one of the simplest and first protections used today. It's akin to not putting the key to your house under the welcome mat.
The rest of your comment is essentially correct, but also simplified. They are not "asking", they are ordering Apple to do it. The FBI does not have the authority under any law to compel that.