Adding all numbers in strict cons-based list is O(n). Adding all numbers in lazy cons-based list is O(n). Adding all numbers in a strict cons-based tree can be O(log n). Adding all numbers in a lazy cons-based tree can be O(log n). Laziness is an entirely different problem from the one the author is talking about.

I see many people here in the sibling replies appear to have also so thoroughly internalized that "cons is for a list" that they appear to have entirely missed the point of the article.