XenForo generally "takes over" a directory, so you can't officially do what you're looking to do. You'd normally install XF in a child directory.
Strictly speaking, if index.php is removed as a "directory index" (making splash.html your index), you don't enable friendly URLs, and you remove all of the rewrite-components of the .htaccess, you may be able to do it. The only alternative would be to bring splash.html in as something like a page node and then change your "index page route" to point to it.