> That's not true, that would mean that webcam video by default would flip any writing and make it unreadable.

That is exactly what happens if I open Facetime on my Mac and hold something in the camera.

To be a bit more precise: There are two streams generated. One is shown to yourself, and this one works like a mirror. The other one is shown to remote participants and this one is not flipped.

Many do, to you the person holding the writing, but not to the people you are showing it to.