I don't think they are talking about mouse events the website can observe. Firefox needs to figure out where the video is and make it's UI on top of that work, there is no observable interaction with the site DOM or its event handlers.
The post says that the video element is told it's being hovered, although it is unclear if that means the rendering context has that element set to hover (thus CSS changes even if pointer events is set to none), or that some internal-to-the-browser state is set to hover for the video.
