This isn't true, Marginalia Search has a crawler. The guy who runs the site writes a lot about the trials and tribulations of running a web crawler on his blog if you're interested. https://www.marginalia.nu/log/ I just checked and my site is listed despite me never manually submitting it. I guess if nobody links to your site, you would have to manually submit it due to it not showing up in crawls, but that's true about any search engine.