I tested a similar color and form-factor Oxymeter, but different display. Low battery indicator comes on at 2.43 volts and it shuts down at 2.36V. AS the voltage drops, the current increases but when it reaches 2.4V the current starts to drop. So there is some kind of compensation for battery voltage in the devices - either a switching regulator, or it increases the LED on time as the voltage drops. Below 2.4V, this regulation starts to give up. Current was about 14mA at 3V and this drops to 10mA if you are logging and the display blanks.
If all these devices are based on the same chipset, then they probably all shut down between 2.3V and 2.4V. There is no doubt they have targeted a device with a high shutdown voltage. Based on the video and the 14mA current of my device, only about 380mAh is extracted from the plain batteries.
Now the problem with the Batteroo-powered device is that just before it shutdown, the low-battery indicator was off - so it shut down without a warning. Without the Batteroo, the low battery warning came on a long time before it shutdown - as it is designed to do.
Is it better to get longer life, or to be warned before the device shuts down? I would prefer the warning.