[RedDevil]

But energy content, though the main factor, is not the only thing at play.

Ethanol will produce slightly more gaseous molecules in burning, so although the temperature would not rise as much as when burning pure gas, this is compensated slightly by the higher volume.
In other words, an ethanol mixture would convert less of its energy to heat and more to power, and even though the power produced will be less in absolute terms, its relative efficiency is slightly higher; you get more bang per BTU.
If your engine is hot enough by itself.

I would not be surprised if engines that have EGR (which adds a lot of heat) generally handle ethanol blends better than engines that do not have EGR.

Apart from that, high octane allows a more aggressive ignition timing and therefore better economy.
Of course, if you mix real bad gas with ethanol to arrive at an average total octane number, there is no positive effect. But if you used the same gas without the ethanol and take the octane hit with that, the FE might even be worse.
It would be unfair to blame the ethanol, the culprit is the bad gas that gets mixed in.