I don't think that's the right analogy. You're comparing tool with a technology. Adobe's tool already did all the hard part for you when you write for Flash. HTML5 can be the same too, once its tool matured enough.
Not to mention equivalent of this implementation in Flash would be equally complex. (No JS/No ActionScript.)
If that's true, then someone's ability to rewrite the tool is academic. And if it's academic, then this isn't impressive. My point is that it's impressive only in the sense that it rewrites the tools.
I can rewrite Adobe's tools in JS 1.5 and CSS3. I went a long way towards doing it - I wrote a whole platform. http://strikedisplay.blogspot.com. Mainly because I was scared of losing the tools I like to have around. But after gaining what I think is a pretty granular, deep understanding of which operations take longer in one or the other (pretty much everything takes longer in JS 1.5), and the trade-offs in other kinds of operations -- the only good thing about HTML5 is that you can force multithreading to suck up an even larger chunk of cycles, a fact that won't be lost on the porn/malware mafia.
Not to mention equivalent of this implementation in Flash would be equally complex. (No JS/No ActionScript.)