No, he promotes him when he kills Ozzel, then says "You are in command now, Admiral Piett." When he says it later, he's reminding him of how he got promoted, basically saying "I can just as easily do the same to you."
And Admiral is a Navy rank, while General is the equivalent Army rank. General Veers commands ground troops and walking tanks (Army), Admiral Piett commands a space-based aircraft carrier (Navy).