I'm struggling to see how he is a good defender if he doesn't do either of these.
There are many ways to defend. Some centre backs attack the ball and the player by getting in to duels all the time. Maguire is one of them, but Varane is not.
A comparison of ground and aerial duels attempted per game:
Maguire 9.2
Lindelof 5.9
Varane 5.4
You can clearly see that Maguire is another type of player. He likes to attack the ball/player.
Note that this includes offensive duels on corners so it is not only defending. Roughly 60% of Maguire's aerial duels are offensive, compared to 70% for Varane and 80% Lindelof.
According to fbref.com going on last season, whilst he doesn't go up for many headers (97)... he does win 75% of them.
To contrast, Maguire wins 77.5% of his headers (from 187 headers), and Lindelof wins 66% (from 105)
The fact that he doesn't go up for many headers could be down to the league he's playing in though, you'd have to ask a regular La Liga watcher about that though.
Yes he wins a good % of them, 77% in la liga and 67% in CL this season and is good in the air, but he doesn't challenge a huge amount. Of course it depends on the league, team, etc, but we can at least say that at Real Madrid, he doesn't win
many aerials. Then it could change if he plays for United.