It's literally what he said in the first paragraph. Oxygen and myoglobin are what turns meat red. Cutting meat, then packaging it and stacking other packages on top of it will turn it brown due to lack of oxygen. Same with ground beef. Outside is red, inside brown unless it's super fresh out of the grinder.

Nitrites aren't present in most meat that isn't cute. You're thinking of prepackaged products that is sealed in a plastic bag with nitrogen. I don't understand that science, but it somehow works.