This explanation makes sense to me until (as seppel mentioned below) I think of a webcam. A webcam (minus any software) isn’t flipped at all—but looks like a mirror if you flip it along the x axis, as many programs do by default.
Take a camera and point it at a friend, now turn it around and point it at yourself.
How did you turn the camera?
You could have turned it towards you by turning it bottom over top and then you would have to vertically flip the image, but you probably would have never even thought to do it.
So instead you chose to turn it left to right so to get a mirror image you have to flip the pixels left to right.
You’re human so flipping a camera upside down seems quite silly but that’s just an artifact of your environment.
That was a really good explanation. In particular the part that we don’t flip left-right, but rather forward-backward and how she illustrated it with the glove.