God was just stating that His demands on the prophets were such that when they spoke the words that God had given to them, the message better not contain anything that wasn't supposed to be there or the prophet would be put to death. Does it demonstrate that God can't fib? Probably not but it does show the lengths that God takes to make sure His message was spelled out exactly as He intended. You honestly think that a God who cannot lie will give a lie to a prophet and then kill the prophet if the lie is embellished at all?
You honestly think there's an iota of logical demonstration anywhere in this thread that because God demands his minions don't lie, that therefore, God doesn't lie?