Found this snippet from Mueller and Splitt about how people keep mixing up HTML and XML sitemaps. It's pretty clear that an HTML sitemap is just for users, not something you can actually submit to search engines like a real sitemap file.
>It's basically a. map of your website for users.The lack of strict structure makes it useless for the direct processing we need. I've always just relied on the XML version for crawling, but if they're finding links through an HTML footer map, I guess it doesn't hurt to keep one. with crawl discovery if there are deep pages that aren't properly linked elsewhere. Not sure if it's worth the extra maintenance for a simple site though.
full read:
https://www.searchenginejournal.com/google-explains-how-to-use-html-sitemaps-for-seo/592508/