Sure! Shutting down certain peripherals can reduce power consumption. The biggest difference can usually be achieved by disabling the USB controller - this is also true for the Raspberry Pi 3 (although on the Pi 3 you would also lose access to the Ethernet controller). So if you don’t use it, turn it off (and it looks like the author didn’t use any of the USB ports, so this would have been an option). Even turning of the HDMI port saves a little bit of power - it makes a difference even if no display is attached. A tiny bit of power can also be saved by turning off the LEDs. I also tried lowering the CPU clock, but that didn’t result in a noticable difference in idle power consumption.
At the moment I don’t currently have a Pi 4 on hand to experiment with, but I do have a Raspberry Pi 4 Compute Module (CM4) on a very minimalistic carrier board and with that I am able to idle at about 1.3 W.
This thread has some interesting details as well: https://forums.raspberrypi.com/viewtopic.php?t=257144
There people managed to get idle power on a Raspberry Pi 4B to a little under 1.8 W. Interestingly, people there report that on their Pi 4’s disabling USB doesn’t do much. If I remember correctly, there were a few Pi 4 firmware updates that optimized power consumption over time. So the different firmware versions can behave differently when it comes to power consumption.
Your 2.3 W doesn’t sound too bad to begin with. Certainly better than the 5 W mentioned in the article.