Almost no one out in the HTML5 community seems to know of this API.
That's because the only demo other than the ones in our big GitHub tree of everything is here: http://todomvc.com
We need more demos and of PureMVC working with various other frameworks. Fortunately, it looks like I'm going to be joining a project soon where I'll be knee deep in JS frameworks and hope be able to more readily contribute to the JS port myself. (I've been primarily a Flex guy for the last 9 years or so).
Is this just a matter of the PureMVC guys having no skill in marketing / awareness?
Yes, sort of.
It's partly because there has been no full-bore champion of it in the JS world as there was in Flex.
But part of it is due to relegating it to a slot in this site that is 'so last decade'.
In fact Mike Britton and I have been chatting about building a site devoted to the JS framework, designed and built with HTML5 tools including PureMVC JS. Something along the lines of the Bootstrap site.