Doesn't seem likely that an advertisement would need to embed that kind of functionality.

What kind of functionality? This is just a generic library for more easily managing keyboard input, JavaScript itself supports the same but in a more clunky way.

How can an add increase accessibility by listening to the keyboard?

Keyboard shortcuts instead of onclick handlers. Unless someone is using Vimium there is no way to use a keyboard to navigate the web without explicitly defined keypress handlers.

How do you envision this to work in practice?

Do I need to read a manual for the ad? Also, the ones that actually are dependant on the keyboard will already use Vimium or something else.