Similar to what Dsell said, but this really pisses me off: "It learned the hard way that HTML5 just wasn't ready to scale as a high-performance game platform."
That's just terrible. Really bad reporting there.
First off, Dominion is hardly a "high-performance game." It's not like they were trying to make World of Warcraft in HTML5 or something.
Second, Goko barely uses anything at all about how HTML5 is supposed to work. So they use the canvas element to draw all their graphics. That doesn't mean it's an HTML5 platform. HTML5 is a huge set of elements and APIs designed to work with your browser to make an interactive web application.
Third, even to the degree that they do use HTML5, none of the problems with the website have anything to do with the platform they chose. Bugs, lack of security, lack of features, ignoring their customers, poor choices on UI implementation, etc... there is no reason at all to think that all of those same things wouldn't have existed had they chosen any other platform.