It's just an incorrect attribution. If there are no transactions, blocks will still be mined. If there are more transactions than can fit in a block, no extra blocks will be mined. If the transaction format changes so that more transactions can be included in a block, the energy usage doesn't change.
It's just a very obfuscating way to describe what's happening. Not only is it not a useful abstraction, but it actively leads you to wrong conclusions to talk about it in terms of energy use per transaction.
