I'm the guy quoted in the article who made the remark about Slenderman as 'the first open-source monster'. I take your point to a degree re. other folk creatures, but the 2 aspects which make Slendy's origin unique and worth describing as such comes down to two factors:
Unlike the other folk monsters, Slendy's creation was a conscious, collaborative, networked process which anyone who wished to could join in - pretty much the definition of open-source as an approach. (A part of Will's interview with me covered how the forums which gently police the canon for each 'pasta bears a striking similarity to the kind of "cocktail party rules" Hakim Bey uses to describe the internal decision-making structures of Temporary Autonomous Zones.That's a very open-source solution to the problem.)
Secondly, it was the first one created after the term was coined! (Though I see that as a minor aspect, honestly.)
Thanks for commenting.