Yes, all defenses will do worse against good offenses. The question is were the Texans unsually worse? I'd like to see more data on how other good defenses did against these teams. After all, the NFL had an offensive explosion this year. Football Outsiders does adjust for opponent strength and ranked the Texans D 7th, not as high as PFF.
Also, the defense scored several points and also gave the Texans favorable field position all season (#1 starting position according to football outsiders). There were also overtime points scored.
From estimated offensive points scored, they ranked 15th.
https://www.teamrankings.com/nfl/stat/offensive-points-per-game
Offensive TDs per game they ranked 20th.
https://www.teamrankings.com/nfl/stat/offensive-touchdowns-per-game
Red zone TD scoring percentage they were 29th.
https://www.teamrankings.com/nfl/stat/red-zone-scoring-pct
They were, however, 2nd in field goals made per game, but as noted above the defense consistently put the offense in good starting field position.
https://www.teamrankings.com/nfl/stat/field-goals-made-per-game
The improved ST's also helped with this.