There's a difference between an iPad and tablets. That difference is something you just can't define. If you go by a dictionary or consensus definition then they're the same but talk to anyone who's used an iPad and they'll tell you it's an iPad, not a tablet.
The post says nothing about why the demand is high and reads like advertising hype. I see nothing special about this Kindle beyond the color screen and Silk, which by the way is nothing more than a glorified browser serving cached content.
I see the iPad and the rest as serving different markets. The Kindle and it's ilk are for book lovers mainly. iPads are for apple fanatics and web content consumers, and the rest are for people who want iPad knockoffs for their cheap price or don't know the difference. I know that sounds really harsh, maybe even ignorant but I don't meant it that way at all.
Seems like whenever a new tablet comes out everyone jumps on the "this-is-the-ultimate,ate-iPad-competitor" bandwagon. Tablet makers seem to be largely copying the iPad as well instead of offering something different. I'd gladly she'll out the extra $400 for the real thing over a sub-par knockoff anyway. These tablet makers need to quit comparing themselves to something they are not (the iPad) and sell their devices on value they offer. Amazon so far has done the best job of this of anyone I've seen but the others need to sell on value offered, not just price. You do get what you pay for.
My views are based on experience and observation. Maybe others have some more solid evidence to validate or detract from mine.