Exactly. CSS does not check that your screen is taller than it is wide. What it sees is a narrow viewport.
// The viewport is in a landscape orientation, i.e., the width is greater than the height.
@media (orientation: landscape)
// The viewport is in a portrait orientation, i.e., the height is greater than or equal to the width.
@media (orientation: portrait)
Maybe there’s something I’m missing, but I’m not sure how knowing the aspect ratio of the screen would help make an element 16:9. If my browser is narrower or wider, that doesn’t affect how I would style the element.