Probably because twice the amount of transactions do require as much energy as half of the amount of transactions or as much energy as no transactions at all, i.e. Bitcoin's energy usage depends on the difficulty of mining a block which is not affected by the number of transactions in a block.

> i.e. Bitcoin's energy usage depends on the difficulty of mining a block which is not affected by the number of transactions in a block.

Only if the transactions do not have any fee. If they have a transaction fee, this indirectly increases the difficulty of mining future blocks (since the miners get more Bitcoin per block, they can use more energy per block before their expected net return is negative, and they are incentivized to do so through competition with other miners; the Bitcoin difficulty adjustment then notices the increase in hash rate and increases the difficulty).

But we can't have twice the amount of transactions because there are also block size limits.

In your example, you are giving concrete numbers. Concrete numbers are extremely useful. If you make a comparison without concrete numbers, it can often be misleading. It goes without saying that concrete numbers can be misleading on their own. It can be quite difficult to nail down rules for honest communication.