I do get, and to some extent sympathize with, your position. But the comments to the article are, in large part, fundamentally addressing a higher level of abstraction than the more narrow scope of the article’s premise. As several commenters have referenced, traversing a tree in a ‘simple’ and narrow manner is addressed at the appropriate level of abstraction using already established mechanisms, particularly in Haskell using Traversable (a typeclass with a standardized implementation for trees). The discussion of Optics and Lenses are more of a side discussion about the syntax and implementations of a more broad set of data structures than merely trees.

I think your comment is valuable in spirit because it could bring a more thorough discussion of the creation or popularization of a syntactically clean and a shift of the idiomatic-ness of such a solution to traversing trees. Leaving the more abstract and general Optics discussion to be a side dish for commenters to discuss as well.

Also, just a nitpick, but traversing a tree is a broader topic than iteration. Iteration implies, both colloquially and formally, performing a computation on each element of a data structure, while traversal is a more general algorithm that could be the basis of iteration, but may also be more akin to a search through the data structure without expectation of performing computation until the searched for ‘member’ is returned from the traversal. So it’s not an apples-to-oranges comparison with your conflation of iteration and traversal, but does seem a little like Canis Familiaris-to-Mammal comparison.