Nope. I worked with Flex and it's MXML files extensively. But the parent is talking about E4X, which was an extension to ECMAScript that allowed you to use XML elements inline with JavaScript in a manner VERY similar to how JSX is used today. It also included the ability to much more easily query and otherwise work with those XML document trees in native JavaScript.

No, writing XML was the declarative part of Flex (like HTML), but AS3 had it’s own XML type so you could do things like this:

var data:XML = <foo><bar>hello</bar></foo>

and then data was an object instance like you’d expect