Mask are physical, three-dimensional objects. If he were wearing a mask - and not just a thin layer of material, but a mask that's substantial enough to make him look like a different person - you would be able to see the bulge or swelling where it begins. It wouldn't be so perfectly flush with his neck that you could only tell he was wearing a mask by the difference in skin tones.
What's actually happening here are just shadows. The interviewer has something similar going on with his neck in the full video, but you can tell a bit more easily that it's shadows with him.