Aren’t the “Demo Sites” that are available for use by the buyers of Generate Press Premium “Block Based Themes”?

Demo content is not the same as a theme. Full-Site-Editing lets everything be build using the editor - including the site's header, footer, and everything in between. I haven't checked what GeneratePress does, but FSE is an approach that is native to WordPress and will be available in core soon.
Everything is a block...
Block-based themes have a completely different structure compared to normal themes. Templates are not PHP files but HTML files. For more information on Block Based themes you can refer to this doc https://developer.wordpress.org/block-editor/tutorials/block-based-themes/