It uses `<meta keywords` and other techniques that apparently search engines like Google do not take into account.
On the other hand the HTML of the page (view-source:https://antipolygraph.org/ ) does not contain any guidance as to what's the page's focus. No header tags are present, everything is `<table>` based and contains deprecated markup and almost all of the links it contains are internal. The markup is HTML4 which might be an indication to Google the page has not been updated in a while.
The website can easily be found in Google (https://www.google.com/search?q=site%3Aantipolygraph.org&oq=...) - it just appears to not be ranked very high as it aged.
Don't forget Google tends to favour newer and updated content to older content. The website also doesn't have a sitemap as far as I know which doesn't help with the problematic markup.