Gotta disagree.
Denver played well enough in the first 7 weeks of the season to average allowing 17 points a game, and begin the road to a #1 overall seed. The Texans D gave up 28.4 ppg over the first seven weeks. They then gave up 30 points in the WC round. If you want to limit it to the last 9 games of the RS, then yeah, the Texans D was just as elite as Denver. Last I checked, every game (especially the playoff games) counts.
Denver averaged giving up less than 15 ppg in the playoffs, and that included playing three of the top 4 offenses (as ranked by points scored) in the NFL.
KC gave up 27 points to NE to end their playoff run (Denver gave up 18 points to them). Seattle gave up 31 points to Carolina to end their playoff run (Denver held them to 10 points in the Super Bowl).
So yeah, when it mattered, there was a clear difference between those defenses. Saying Denver's D is only called elite because they won the SB and we would have said the same thing about Seattle or KC had they won is akin to saying Denver's only called World Champions because they won the SB, and had Seattle or KC won, we would have called them World Champions too (in other words, accurate, but meaningless).