Followed these exact instructions with iFixit OEM screen replacement, and the front facing camera no longer works. @mattmontero explained the reason (above):

"These instructions neglect a key point: there is a protective cover over the holepunch camera opening (with the OEM part), and if you don't remove that before installing the screen, you will not be able to take selfies."

It sure would've been nice to have that detail included in the instructions. Now I've got a Pixel 6 Pro with no front-facing camera.