Huh, so it does exist! I had to scroll through an almost endless list of tiles, but I was able to find it. (I tried a second time to count how many, and it wasn't there that time.)

Turns out it's also possible to intentionally navigate to it: app menu (top left "hamburger" icon) -> "See All Programs" (under "Programs and Features" group in menu) -> "Your Dash Buttons". And the UI looks just like dash buttons. With buttons that say "Buy now" rather than adding to cart.

It'd be great to have the ability to somehow pin it in a more prominent / convenient location if I were seriously planning to use it.