> To me, at least, it's quite easy to imagine that the founder of the project -- the same person who posted this Show HN, AFAICT -- may have told the PJON community "Hey everyone, I just posted a link PJON to HN; here's the URL if anyone wants to join the discussion ..." or something else similar to that.

Coming from the project's Gitter that is exactly what happened. I am pretty sure some of us over there do not have an account at HN. As far as I am concerned, this is just the third time I wrote something here since creating my account in 2017.