If the hierarchy is "public visibility", then I believe the dark net to be more visible than a private WhatsApp group.
Is there something meaningful (in terms of linear continuity) on the y-axis of a forest? Or are you looking for a continuous function to apply where the metaphor isn't asking for one?
And there really is nothing cozy about data-hoarding closed mega-corp software like Discord and Snapchat. As convenient as they are, 'cozy' is not the word.
The writer likely just thinks "LinkedIn and Facebook == Bad!" and "My two Discord servers with my friends == Good!" and just made huge generalizations from there.
... but not for long, and it's not the expected common-case continuously-observed scenario when one walks in the woods. The point is that such a thing is a transient, unstable configuration at one layer and the expected configuration at another layer.
Your comment seemed sensible to me, until I realized that it's likely because we are both old and think about trolls as being on 8chan.
But if you look at pretty much any time since about 2015, the most notable trolls have been large public figures with an extremely wide reach, saying the same sort of things that get a rise out of people on 8chan and Reddit. To take the most recent example, Elon Musk's upending of Reddit can be seen at least partially as an instance of a large public trolling. On a more distributed plane, phrases like "own the libs" are common enough in the public consciousness to be used by angry 45-year old commenters on Facebook who have never been to 4/8chan.
That is why trolls are at the top of the diagram, because it's a diagram as of 2023 -- whereas you're thinking of trolling from 2013 or maybe even 2003.