Canvas was around before HTML5 too (AFAIK, Apple invented it for Dashboard), but it's still generally classed as an HTML5 thing. Much of the role of HTML5 is standardizing all the disparate technologies that are already being used to make sites but which aren't really regulated by the HTML standard.
As long as CSS opacity is not explicitly specified for any SVG element, I have found FF is not very slower than Chrome/Safari. But the moment opacity attribute is added even for a single element (even if the value is specified as 1), FF really slows down.
So I tend not to use opacity in my JS rendered SVG especially if there are any animations.
Although in a lot of ways Canvas is easier to work with, SVG has some advantages for user interaction because it takes care of interaction logic for you. For instance, you can register a callback for clicking or hovering on complex shapes, whereas with Canvas you'd have to calculate the hit detection geometry yourself.
I read the article with some interest remembering what it used to take to work with SVG (the Adobe plug in---free, but still). Just for grins I copied one of the examples to my editor, EditPlus which uses whatever version of IE that you have installed on your system. To my very pleasant surprise, the example worked. This should open up a very large box of goodies of interest to me. I spend a lot of time working with chess and HTML. If this could simplify my code in any significant fashion then I'm all for. It will be interesting to explore.
The last time I played with SVG (on January) it had a couple of ugly bugs like crashing Google Chrome with many nodesin a graph and leaving trails when moving an object (a Hello Kitty for my daughter!)
Personally, I don't think it is. For most use cases, Raphael is a better choice for its browser compatibility (namely IE up to 8) and approachable syntax.
Of course, it's also beneficial (IMO) to have a grasp of how the underlying language works even if you use it through a layer of abstraction. Overall, though, I'd use Raphael unless you have a reason not to.
EDIT: I forgot to add that knowledge of SVG is useful for editing images in a text editor, such as when you want fine-grained control or you want to manually remove some of the cruft that graphics software tends to add to files.