I dnt know about the engine, but Yes you can develop games in flex.
Flash and Flex both use actionscript. There are no special features in Flex that would make it more suitable for developing games with compared to Flash.
Of course you can build games using Flex, but why would you?
Well dont forget that Flex means "the framework libraries" for building rich internet applications.
In other words, flex provides a rich set of controls, container and other elements, everything based on ActionScript 3.0. so, the better response to your question would be:
You can make games using the flex framework, but really you need to know ActionScript 3.0 because if you think to do a game, hardly you will make all elements using flex framework, maybe some elements like buttons, checkbox, panels, etc will help you, but most you will need to make using pure ActionScript 3.0 and maybe some help of Flash IDE for design of your elements.
In resume: Of course that you can build games using flex, but don't forget that flex is only a framework, MXML is a markup lenguage that provides an easy way for layout, but finally everything is ActionScript.