Trump is a bit overweight. His head/neck isn't separating from his body. He is tilting his head and the extra bit of skin/fluff on his neck rises up revealing more of his shirt collar. This happens with all overweight people in a collared shirt.
It's not a deep fake. I can almost buy the theory that it isn't a concession speech (though I 99.99% believe it is), but this is definitely not a deep fake.
To me it doesn't matter if it is or isn't a deep fake. That kind of video is insufficient, regardless. We have no way to know what's really going on without being able to see him "live."