No, not at all. I cite non-academic sources (light table, Bret Victor) all the time in my research, however keep in mind: you are writing for your readers, and you understand their mindset. Writing for an ICFP crowd would be a bit different than writing for a hackernews crowd, so you would compare against more academic work (especially since the program committee are academics) and not so much industry projects.

A 12 page paper can never really be "complete," so optimize for the intended readership.

Well, "published" here is used in the academic sense, so yes.

It's not about some random blog posts or presentations, if that's what you mean.

To some extent, there is such a rule. But not all academics are perpetually 2-5 years behind the times by only getting updates from journals. Bleeding-edge researchers go to conferences, which can cut the latency down to as little as 6-12 months.

PL doesn't have much in the form of journals; we,rely mostly on conferences for communication, though I think that's changing a bit.

"published" != "on the web somewhere".