We don’t like to outright say someone is wrong, but, well, Google Analytics doesn’t give you the full story on the subject of bounce. Because GA doesn’t measure every interaction on the page like FullStory does, it has no way of knowing how long someone spends on your landing page. It can only determine active time if a user navigates to a second page; then it compares the timestamps between the two.
This method is, at best, fairly inaccurate. If you have a landing page loaded with information, or an increasingly trendy single-page site, the user could spend minutes or hours reading your content and GA would still consider it a bounce, just because they didn’t navigate to any additional pages.