That part is wrong. A large part of what happens on a rugby field is scripted, the phases are set by the coaches and rehearsed on the training pitch, the main difference is that Rugby is a live game, so the instructions and sets are situational and the players have to use their brains and special awareness in order to realize which scripted play they are supposed to use, they also have to think about defense and offense. In the NFL, the clock is stopped and the coordinator tells you what to do, the one complicated thing is that they have more plays but even then it doesn't really apply to running backs, it's only really relevant for offensive linemen and quaterbacks who need to know everyone's playbook.