Each defensive lineman is responsible for “filling” one of the gaps, and the linebackers are expected to fill the gaps that remain. Generally, on each down one linebacker and one cornerback or safety is given an additional responsibility as the primary and secondary containment defenders. Their role is to stop the rusher from getting outside of the defense, forcing the rusher in to where defensive traffic is heavy and away from the sidelines.
The linebackers job is made easier or more difficult by the success or failure of the defensive linemen. If defensive linemen fail to fill their gap, and a running lane is created in their gap, the linebackers either have two gaps to fill, or a far wider gap than the defense intended.
Again, this article states the D line was ‘reguarly blown off the line’ on Sunday. I’m not sure that’s true. It didn’t happen until the final drive.