I hope he is referring to the intermediate/MOF missed targets as "underneath" here.
Otherwise, this is bleak.
On if the 49ers defense limited QB Bo Nix’s downfield passing
“I think to your first point, and we had seen this on tape, they did a pretty good job of staying on top of some of the deeper throws and [that is] part of what they’re trying to do defensively. So I think coming away from that game tape, I think the feeling overall was just more yardage underneath in different ways that I think we feel like we could have taken a little bit more advantage of.”