For displaying the Google indexed pages the SEO plugin just fetches the Google Search result page.
But as Google is blocking more and more bots from accessing the search this feature is becoming more and more useless.

I am not sure if there is a API to access this information (apart from how the premium plugin does it), so I don't know what to do.

More information: https://forum.piwik.org/t/google-indexed-pages-zeigt-0-an/25835/8