Both. He knows how tall he is, he has been a revelation in defence for United. He's been winning headers against taller strikers when he had no right to win them. He missed a major chunk of the season and it showed on the team what United missed.
If he was so weak in the air, City would have certainly targeted him in the FA cup final especially when they were trailing for the majority of the match. He won headers and defended brilliantly against Haaland.
A CB can also be slow and a great defender. Not that Martinez is slow but his reading of the game is excellent. If he stays injury free, you will see how United's fortunes improve dramatically.