The goal of pre season games is not to win them..... now is the time to experiment with team lineups and tactics. And for guys like Mkhi to show they have more in their game.
If Emery is honestly considering Mkhi as a CM option alongside Xhaka, Torreira, Douzi, and Willock then I think it made sense for him to play CM today. However, I think it is pretty obvious from Mkhi's entire career that he does not play CM, so I'm not sure what would be the benefit of trying Mkhi there when we have Willock, Burton, and Olaynika pushing for minutes in the first team.
Preseason isn't just about winning, but making decisions that are hard to explain and clearly lead to losing is still a problem.