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.