I consider this a bit of a frustrating pseudo-myth. It's true canvas is slow compared to lots of native drawing, but its usually presented as a false equivalency issue. Canvas didn't set out to replace native drawing of hand-crafted OpenCL. It's an alternative to cross-platform graphics on the web, where you get Canvas or you get Flash (Or SVG or hobbling together colored DOM elements to animate while crying and eating ice-cream out of a gallon tub. We've been there. Don't lie.).
What's worse, looking at the videos demoing his Mac app[1], they don't even look like they'd need canvas levels of performance. They look like they'd work just fine in SVG. What's wrong in this case with the very good performance across many platforms (even tablets) of the SVG-backed RaphaelJS?[2]
Unless his app can do things he'd rather not demo, I'm guessing this post is moreso post-hoc rationalization of picking Mac as his preferred development platform. At the risk of being a bit rude, it's worth noting that he doesn't bother with all Desktops, just Mac, which causes further suspicion that this is really just a rationalization piece and not about performance.
(Unrelated to the post at hand, this canvas pseudo-myth also upsets me because I spend big chunks of free time helping people with their canvas work, and its almost always an issue on the programmer's part. This is fine, I've never fault a programmer for writing less-than-optimal code, but often programmers tend to contribute their voices to the chorus of "Canvas is slow", regardless of looking for fault prior to their declaration.)