Both of these guys are 3T and can penetrate.
Donald leads the league in sacks almost every season.
and then think about the fellow teammates besides Donald on the DL .. Suh and Brocker. Two guys that are known more of their run defense than pass rush.
Especially with Suh at this stage and age of his career. That further helps out Donald when it comes to team defense vs the run.
In Fletcher Cox, you have a 3 tech role that has always been notable run stuffer, even though he doesn't always show up as often as a pass rusher as he used to
And you look at Philly's LBs and they are nothing special. Decent at best.
But they are decent enough to finish off what the DL does in the trenches to allow the backers to operate and keeps them in ranks among better run defenses in NFL
I believe the key has always been with Philly does in their DL, albeit Philly has been going thru 1 tech guys the last few years, Hargrove is the currently the most recent
added one, and in the past it's been injury prone Tim Jurnigan.
But it does matter the level of performance of a dominant 1 tech. He cannot solo make the run defense better by himself as it does go by overall team performance
but as a guy that demands double team, he does help a lot with the dirty work, occupying the lanes for "capable" LBs to flow freely to the action and lessen the traffic.
And also if we look up the contracts of the better 1 techs per FA, and see if they don't get paid for such terrific clog lane dirty work ...