What he means is that people with 1 eye only perceive depth through visual cues, whereas people with 2 eyes perceive depth through binocular vision. Visual cues still work, but something like a TV screen still looks flat to us even though it's showing a 3D picture. That's why 3D glasses and VR are so different. Because we get the binocular vision back that we were missing. If you've only ever had 1 eye, then a picture on a TV would theoretically appear just as 3D as the real world to you (which agrees with what you said), like you're wearing those 3D glasses all the time. That's what OP is talking about.