More context:
-4th in forced fumbles (19)
-T7th in sacks (42)
-Held opponents to a 66.3 and 69.9 QB Rating in the 3rd and 4th QTRs
-1st in pass deflections
-13th in rushing TDs allowed (11)
-12th in passing TDs allowed (22)
-7th in Passer Rating allowed (79.0)
Some of their problem areas (penalties - 19th, YPC 28th) are issues the team focused on fixing. So far this year they are 4th in YPC allowed (3.4) and 4th in fewest penalties (11).
There were some inconsistency issues with last year's young team as well (as noted by FBO's variance mark:
2017 DEFENSE EFFICIENCY RATINGS | Football Outsiders), but they were an above average team in the NFL. I think you kind of point out several of the reasons why those yardage numbers go up. The Saints built leads and went into prevent defense mode. They focused more on eating clock than they did preventing yardage. Now, the effectiveness of this strategy can be debated, but as a team defense they were very good, especially in comparison to recent New Orleans teams.