[NSFW] self hosted video block does not seem to have thumbnail facility

I am new to WordPress and am using the block editor not what I think you call the ‘classic editor’. I have a page with just a heading block and a video block so that Google will see the video as the main content of the page which seems OK except Google will not index my page because it has no thumbnail for the video. I have asked ChatGPT how to do this and it wants me to jump through many hoops to make a thumbnail visible to Google (and linked to the video) but invisible to users, and even then I am not sure it will not upset the rule about being the main content of the page. Is there not a way of inserting a thumbnail into a self hosted video block? If not shouldn’t whoever created the video block go back and allow for this?

The page I need help with: [log in to see the link]