Consider the guy highlighted by the red circle in the following image:
to describe him, is each of the following expressions correct and idiomatic?
He is the horizontal position.
He is in the horizontal position.
In other words, is the word "in" required or omissible when talking about position?
You would say "He is in a horizontal position", rather than "the horizontal position" which I think is incorrect.
For clarity, all of the people depicted in your graphic are in a horizontal position. You could also say "all of them are laying down" or "all of them are in a laying position". As @user105719, it's possible to describe someone as supine if they are flat on their back but I would say its unusual to hear this in everyday language - I think most people are more likely to say "he's lying on his back". The other people in the graphic can be described as "lying on their side" or "lying face down", in case this is useful to you!
He is a person, not a position. So you can't say that he is the horizontal position.
You can say "he is horizontal", and that is probably what most native speakers would say if they are thinking about it enough to keep their thoughts concise.
