Actually, technically, they are using Scaleform, which is a framework that makes function hooks available to a swf file. Basically, you can write Actionscript that calls functions from within the game (as long as you are given access to them through Scaleform). "Flash" is technically just a platform that uses Vector graphics and allows Actionscript modules to be attached.

As for Actionscript being slow and bloated. Well... yeah it kind of is, but only in comparison to compiled languages and specialized scripting languages like Lua. As has been mentioned before, good programming practices can make up for a lot and significantly streamline Actionscript code.

What's cool about Scaleform is that, except for the fact that you can only interact with the game through the exposed API, what you can actually do with your addons is virtually limitless. You should have full access to any libraries, effects, tweens, etc available in vanilla Actionscript.

I do a significant amount of my own work in Flash and Actionscript, and while there are many more well versed than myself I'm always happy to share what I know, so you can add my name to the Learn Actionscript "in like an hour" sensei list.