That example is still an undirected cycle so not a polytree and so, by the reasoning of the author of tfa not kosher for reasons they don’t really explain.

Honestly I think the author learned a bit of graph theory, thought polytrees are interesting and then here we are debating the resulting shower thought that has been turned into a blog post.

That was my impression as well. There's pretty much no argument for why a DAG is worse than a polytree.