To nitpick: at rest, muscle cells use fat over carbs to produce ATP. It is only during effort that the cells start to use more carbs than fat. The rest of the post is, to my understanding, correct.
As intensity increases during exercise, the body starts releasing muscle glycogen. If previously the muscles were mostly using fat (because no exogenous carbs were available), now that endogenous carbs are ready, the muscles will use them instead. You cannot, however, reduce muscle glycogen use by supplementing extra carbs.