They have played exactly 4, all from the SEC. That's a bullshit stat, and you know it. When teams play EACH OTHER and both have a shitton of fumbles and picks, and their offense is inept, it makes the defense look good on paper, but you can't sit and tell me that the Tennessee and Texas A&M teams we've seen this year lived up to the hype. Their head-to-head in week 5 or whatever was absolutely pathetic and full of obvious mistakes, but yet both defenses look good statistically in that game.
Add to the fact that every single one of these "top 20" defenses has played a division 2 school AND a g5 school, and it's pretty obvious that the numbers are skewed. I'm sure Ohio State would score 800 points and record 7 safeties against Chattanooga, which would make us the best team in the history of the universe on paper, but instead we opted to schedule Oklahoma.
While your statement is technically true, let's not act ignorant to how those numbers came to be.
But since you want to cherrypick, OSU has played 3 top 10 defenses this year already, Alabama has played 1. Checkmate.