Ah yeah you just reminded me of the rabbit hole I went down a couple years ago.

Yeah I had to flash the firmware on the Anker dongle, following this guide: https://forums.macrumors.com/threads/dp-usb-c-thunderbolt-to...

I just tried streaming 4K through the Apple TV app on my M1 Macbook Pro and no issues, so I'm assuming HDCP works.