Ya know, something about this statement just didn't add up for me. So, I went to Pro Football Reference and checked into it.
Tomlin was the DC in Minnesota in 2006. According to Pro Football reference, if one is only looking at passing yards allowed on the season, you are absolutely correct, the Vikings were dead last in that category. You know what else is as important (in my estimation more important) as total passing yards allowed? Passing TDs allowed. Guess what team finished in the top 5 in that category in 2006? The Vikings finished number 4 (14 spots better than the Steelers), allowing only 15 passing TDs on the year.
As a bonus, since total yards allowed are the determining factor for you, in 2006, guess which defense lead the league in rushing defense. The Vikings allowed only 985 yards to lead the league. Now, since I say that TDs allowed is as important (more important) than just yards allowed, it's only fair that I tell you that the Vikings allowed 9 rushing TDs that year to rank 3rd in the league (a spot they shared with Pittsburgh).
How about points against for the year, you say? Well, here the Vikes were definitely average; coming in at 327 points allowed, which put them at #14. Twelve points and 3 spots lower than the Steelers defense.
So, yes, Tomlin's Vikings defense was the worst defense in the league in terms of yards allowed on the season. However, I wouldn't say his body of work that year was subpar or made him unqualified for the job. In fact, it made him exactly what I believe this team needs in it's next HC (albeit on the other side of the ball), a young, up and coming coach, full of energy with a proven track record of success.
Now, I'm sure you were unaware of these statistics when you cited the one stat that put Tomlin in a terrible, unqualified light. Otherwise, we'd have to assume you were ... what's the opposite of virtue signaling?
https://www.pro-football-reference.com/years/2006/opp.htm