"The vast majority of Bitcoin’s energy consumption happens during the mining process. Once coins have been issued, the energy required to validate transactions is minimal."

Honestly, that doesn't seem particularly well-informed. I wonder how this person thinks transactions are committed to the blockchain, and how this is separate from the mining process?

Unless they literally mean "the energy required to consult the ledger of previous transactions", which doesn't seem a useful metric.

The quote you referenced makes the distinction between committing (mining) transactions and validating. The energy used to validate transactions is indeed minimal.

The energy used for validation is a useful metric because it's how the bitcoin network can afford to stay decentralized.

The confusion comes from how the miners are compensated for their energy burning. Currently, it's mostly by the block subsidy, which is independent of the amount of transactions in a block. But in a decade or two, that will be dominated by transaction fees, and we can actually attribute the energy cost to the transaction amount.