Pretty simple, they have 9 upperclassman and 0 freshman starting.
College kids grow a ton in 2 or 3 years. They also develop skills, even if they didn't have great defensive coaching prior to a change.
Amazing the same upperclassmen gave up over 500 yards to BYU before they got a coach who "simplified the schemes" and is a great motivator.
For the record, Nebraska players aren't executing the "simple things" right now. That's ultimately on coaching, but you can't "dumb down they system" further than it already has been.