He's an academy player that arguably got a chance beyond his actual abilities. It has definitely happened before. Scotland isn't all that great as a national team, and he's had to play CB a fair bit on top of that.
He's a decent allround player. He's fairly strong. He's big. He's a presence in both boxes. He times his runs into the opposition box quite well. He has a mean shot on him from time to time. His long range passing is actually very good, which makes it weird that his short range passing is so abysmal.
I've seen him lose countless foot races. He's not fast by any stretch of the imagination, especially not on the first few yards. He also has absolutely no agility.
I may have misjudged him in terms of stamina, but I don't see him as a Fletcher that is all over the pitch, or a Fred - he has never stood out for me in terms of working hard. For all of his faults, one could clearly see Fellaini as a very hard working player. McTominay hides much more, and he's not close to being as physical a threat as many players his size - think Fellaini, Vidic, Rio, VVD, Ibra, Haaland, Pogba, etc. Looking at central midfielders, Rice is much more mobile. Goretzka is much faster, and then you have previous players such as Yaya Toure who were immensely athletic.
I genuinly believe his physical appearance gives him more of a look of being athletic than actually being athletic. That being said, it could also be semantics and how you define it. Him clearly lacking agility, is slow on the first few yards and only decent in terms of top speed, not having a great leap and only average in terms of strength for his size, I just don't see him as being rightly labelled athletic. He is probably one of the fittest in the squad, however, but is that being athletic? There were rumours about him finishing a 5K run in 16 minutes and 1 second, which would be incredibly fast - but Maguire doing it in 18 minutes also means that he must have been running at full speed for 5K, which seems bizarre.