USB External Monitor forces MacBook Pro to use integrated GPU only

It is actually WORSE than you describe. That display does not use any built-in display acceleration Hardware.

That display is a DisplayLink display. It creates an un-accelerated display buffer in RAM and writes its data there. To update the screen, the data are sent out over USB and passed to a DisplayLink chip in the external box (or hidden in the display housing) and turned back into a "Legacy" display interface.

The bottom line is this is a good choice for brute-force display of un-processed screen data, but a remarkably poor choice for anything that requires filters or modification or fast updates or smooth motion.

Get a "real" display and live happily ever after. DisplayPort or ThunderBolt interface is vastly superior to Legacy displays {HDMI, DVI, Analog VGA}

The features page for the display you bought points out that it uses DisplayLink Drivers, and will not work without the DsiplayLink driver added. THAT is the key difference.

DisplayLink is a separate company and has their own web site that tells how their stuff works.

<<so does that mean, any usb-c based external monitors that use 1 cable to both power and transfer data will behave in the same manner you described? >>

No, that is not the determining factor. That is a DisplayPort display that uses DisplayPort software rather than any built-in Hardware. DisplayPort stunt-boxes have been available as add-on display interfaces, but have never been fast enough for the items I mentioned, because there is no display acceleration hardware.

The makers of the display you are using have figured out how to get the power consumption way down, so that it accidentally fits on a USB-C cable. But there are other USB-C displays that do not use DisplayPort but use a built-in graphics chip instead. In most cases, if you use an external display, you need to plug it in to AC power.