I think you misunderstood the docs. They said
Extensions may not use keyboard shortcuts to power functionality.
They mean hotkeys like ctrl + x, because twitch already uses hotkeys and they don’t want yours to conflict with theirs. Another part of the docs say
When a viewer clicks into an extension, Twitch sends focus back to the player, to ensure that keyboard shortcuts for the player continue to work. However, if an extension asks viewers to click on a form field element (for example, “field,” “select,” “textarea”), and the viewer does so, the focus stays on the form element.
You can use any inputs you like.