Is about-us.php some sort of custom page template file? Because stuff like metabox definitions have no place in a template file regardless the context. I am talking about the general good code practices. You should separate your functions, actions and the frontend output, otherwise you are just creating a mess of a code.
add_meta_boxes action itself can be in a theme, if those metaboxes are used for design (I mean that messages literally tells you that), but you should place it in a functions.php or some other file where you are placing your actions. Not in a page template file.