No you've got it backwards.

Robots.txt absolutely stops Google from scraping your site.

But they can still learn enough by scraping other sites that link to yours to build a concrete picture of the contents of your website and they will use that info to populate search results that link to you.

If you don't want to appear in search results, then you need to tell Google which pages to hide, and to tell them that you have to allow them to scrape your site.