Honestly I dunno why it was the case for me. What I know is that I thought I'd save power pushing pulses over copper rather than using a radio. I didn't. In fact it was consistently about 40% worse than wifi.
That might be unique to the RPi hardware I used. It might not.
The Pi does Ethernet with a seperate and pretty power hungry chip.
If the ethernet cable is disconnected, that chip mostly powers down.
I think it's a Pi related thing rather than being inherent to Ethernet. In fact, if I were to guess, the energy per bit per meter of ethernet is probably far far lower than WiFi.
That might be unique to the RPi hardware I used. It might not.
Maybe I should have written that article. :)