You can make the argument, and Football Outsiders, among others, did in 2017. If the Eagles won the Super Bowl in 1991, they would probably be considered the best defense of all-time. This is Ryan's defense without Ryan, run by Bud Carson.
The Eagles had problems on offense, and that is not on the defense. Rich Kotite may be the worst coach of the past 50 years. Cunningham went down the first game for the season. The O-line sucked balls, one of the worse in the league. They played on more short fields than any team. They had to play, just in the East, the team with the best offensive DVOA (Washington), the fourth (Cowboys), and sixth (Giants) twice. Only the 2002 Bucs have had a better pass DVOA, and the 2000 Raven's a better run DVOA. Teams ran a ridiculous low 2.97 ypc. QB's only completed 44% of their passes against that team. 55 sacks, 43 forced fumbles, and 26 interceptions. That is insane. I also believe they led the league in fewest points, fewest rush yards, and fewest pass yards.