Maybe I'm thick. The article is dated February 2014, it says flexbox "currently lacks browser support, mainly in IE (version 10 has partial support and full support won't arrive until version 11). Chrome, Safari and Firefox have support but with browser vendor prefixes."
IE11 came out in preview in June 2013, released officially in November 2013, and at least IE and Mozilla are currently supporting it unprefixed (though Mozilla's multi-line flexbox fails last time I checked). This article is over 3 months out of date, if not more, but being presented as new and current.