A key part to this is: "Since srcset and sizes are only available in the latest version of Chrome 34 and Opera 21 utilizing Scott Jehl's responsive poylfill in the meantime will allow your site to be retrocompatable until IE and Firefox adopt the latest specs."
"How we fixed responsive images" for the latest versions of two browsers.*
It's true - but since it's been adopted into the official spec it's only a matter of time until it's adopted by all major browsers. Using the picturefill 2.0 polyfill mentioned in the article makes your code retro-compatable. You can see their full support explanation here: http://scottjehl.github.io/picturefill/#support
They have a fallback, don't be mean
I didn't say that to be "mean." It's just an important note to people when they see cool new things like this to know the browser support.