Don't think so. Downwards, not straight down. You can't keep your elbows straight down when you keep your palms lying flat in a horizontal position unless you twist your arm strongly. Sun stylists keep their elbows down in the beginning movement, they lift their arms while keeping the palms vertical facing each other. Yang stylists usually do not.
So just why didn't YCF/ghost writers include any of that in his/their 10 essential points?
Why don't you ask YCF's ghost? BTW, the essential is (5) "Sink Shoulder, Drop Elbow." As Glenn pointed out, "shoulder" in fact the shoulder blades. This is an aspect connected to (2) "Contain the Chest, Raise the Back". If you keep your chest hollowed and your armpits opened, there is no way you can keep your elbows hanging straight down to the floor when you move. The key is to keep a feeling or slightly press this area downwards and moving your arms from this position. If you understand to work with (2) and (5) properly together you will achieve a feeling of your arms as they partially were moving by themselves as you learn how to control the lifting and sinking of the limbs from the spine, which is one of the main points with these principle in combination. People usually say that they use qi when they achieve that feeling of the limbs raising and sinking by themselves, but this feeling is very much a function of body connections in a quite physical way, that you learn to move in a certain way, learning to use your body in a specific connected way that results in a whole body generated movement that feels light and effortless (and the limbs can feel very heavy at the same time). But I wouldn’t call this Yang style specific or a secret, if I would regard something as "secrets" specially for Yang style, I would put YCF's 9th and 10th point first on the list. If you don't understand and use "chousi” throughout your movements, your Tai Chi could not be called Yang.