Also, if I remember correctly, the default browser in Windows 7 is IE9. It makes sense to support default browser in the most popular desktop OS.
Some of older browsers, released in 2012-2014 support flexbox only with vendor prefixes, but the article doesn't has rules with prefix. As frontend devs tend to copy the code without much thinking, we cannot expect that they will add the prefixes or fallback code themselves. So it will be author's responsibility for web sites being less accessible in different browsers.