Mind you I spent a lot of time working with Haskell in an academic environment and never understood the first thing about category theory. Especially those bloody diagrams that were supposed to be proofs.
It is entirely possible to understand functional programming and abstraction without understanding any category theory...
Not sure if joking.
Here's an analogy. There's an apprentice carpenter who has been given some tools and as far as he is concerned the tools he has been given are all of the tools in existence. It's quite clear that if he was given a new tool, he could immediately establish whether it was useful or not. He could not establish, however, that he has access to every useful tool.
From his account, I don't know whether CT is like a hammer that he is using upside-down (needs explanation before it's useful) or a banana (never going to be useful to construct anything).
Saying that understanding category theory can give you a greater understanding of functional programming doesn't imply that a lack of understanding of category will cause problems in the understanding of functional programming.
Our difference is that I do not see anything in the thread parent that contradicts the article, and I don't think he's trying to convince you that CT is either a hammer or a banana, just that professional carpentry work is possible without it.