I've spoken about this before, but I'll say it again. When you find yourself on an offensively dominant line, such as the Marner-Matthews-Bunting line, the matchups you get aren't trying to score on you. They're trying to shut you down. They don't care about scoring goals when Matthews and Marner are on the ice, their sole focus is shutting those two down. This makes playing "defense" for players like Matthews and Marner much easier because they hardly have to do much defending, at least compared to most other players.
So posting these defensive metrics are just so so misleading because they really remove the context of the roles that these players play and the matchups they get. McDavid gets the same effect of Matthews where his matchups are just trying to shut him down, but neither one of these guys are used in defensive roles whatsoever, so while metrics might indicate one is superior to the other, the reality is that neither one of them have much defensive impact in general, because neither one of them are actually used in situations where they are actively defending instead of just "go out there and try to score".