Don't use WP All Import to import theme demo content.
Use https://wordpress.org/plugins/wordpress-importer/ instead.
Theme demo content is already in a format that can be read by the default WordPress Importer. It's not flexible at all, but that's OK, you don't need it to be. The WordPress Importer already knows how to read theme demo content XML files. It will automatically place the theme demo content in the right place without you having to set up your import options like you would with WP All Import.
The point of WP All Import is to enable you to import an XML or CSV file in any format to any place in WordPress. This requires configuring options inside of WP All Import to get your data into the places you want it.
