FYI I’ve added this plugin as well https://wordpress.org/plugins/export-media-with-selected-content/ but then when I export the XML file only contains authors, no posts lol
Migration from some kinds of sites will be less reliable than others. I’ve even had trouble migrating some WP sites where 100% compatibility would be expected. If the same image file structure was maintained, you could manually copy the files via FTP. If the structure changes that could be a very tedious task.
If the export tool is supposed to be compatible with the WP importer but it’s not working as expected I suggest seeking help from the exporter’s dedicated support channel where its devs can help you.