"However, the old 5V processors did not have fans on top, so "less energy" is hardly the case."

But you're comparing apples & oranges.
When was the last PC processor that didn't have a fan?
Early 486s, I think.

Look at how much extra stuff they've packed into the processor since then!

So, although the actual power is higher, the relative consumption may well be reduced;
They probably use less energy than if they'd carried on at 5V