I believe that rulebook quote is, as they say “fake news”. However, teams are supposed to be on the field ready to play 10 minutes ahead, and that would mean on the field for the anthem. So you can’t sit it out in the locker room, and their own code of conduct in the compliance manual states this political behavior is unacceptable.
The free folk do not bend the knee. Men do not bend the knee. America does not bend the knee. And we know no king but th......ahem. got carried away there. Anyway, you get my drift, right NFL?