It is an easy play to make, but the situation--skating back towards your own net with a man in your pocket and another to your inside--is not one you encounter a whole lot. And that's the whole thing, you often see Miller make mistakes where there is a "simple" solution, but the plain lack of in-game reps cause him to panic. It's simple once you've encountered it or if you're just naturally gifted, but for guys that are learning, it's not so easy.
That's a lot of what's wrong with the defensive aspects of his game; he panics. But it's from a lack of experience, which is why I'm not particularly concerned. The only thing that he needs to worry about, I think, is that he's going to have to continue learning as he starts his pro career. He's going to be at a big disadvantage because when he gets to the AHL next year or the year after, he'll be learning while the other guys are way ahead on the development curve. It will be a matter of how quickly he can adapt.
EDIT: To add to this, what he attempted wouldn't be unusual for a forward in the offensive end. If he was skating towards the net, lobbing a saucer cross-ice wouldn't be a totally terrible play. If he was skating back to the blue line and tried that cross-ice saucer, he's at least skating back if it's picked off. In neither instance would it be a great play, but you see forwards in the NHL trying to get cute with those saucers across the ice all the damn time. Which just reinforces that his instincts often times default to what he would do as a forward.