Well, traditionalist pixel artists often hold that the means by which the image is created is what makes it pixel art (or not). Chrono Trigger's graphics are pixel art; each pixel has been individually placed. Aquaria's graphics are not; they have been digitally painted, using tools that affect surrounding pixels and alter their properties automatically. I'm not entirely sold on that definition, but on the other hand, it neatly removes the problem Glamador raises about aesthetic; essentially, by that tenant, it doesn't matter how it looks, just how it's made.
I don't think "high-end pixel art" is a particularly helpful term. Assigning a value to art like that has never sat well with me. While there's no denying that the screenshot shown is technically very adept and impressive for it, I personally don't think it's inherently better than, say, Flip Flop Flyin' (which is visually much, much simpler, but much more sophisticated in its use of pixel artifacts). It's just a different, more traditionally representational approach.