Strictly speaking, buttons aren't typically hypermedia controls. Haven't read the paper yet but the only controls in HTML are links and forms. Buttons can be nested inside those as actuators but aren't controls unto themselves since they don't meet that functional requirement of sending a request to a URL and inserting response content into the viewport.

EDIT: It seems the authors also consider images and iframes to be controls since they also make requests and add content to the viewport.