Sure, doing hard things is a good thing for satisfaction and character building. That doesn't mean a math book that's harder to understand is better than one that's easy.
I think Finnegan's Wake (mentioned in the article) is a good example. It's difficult and good, but for different reasons. Joyce was being expiremental. He wrote from inside a characters head, a POV style that's very common in modern novels. So, the expirement worked.
So, in a sense, it's hard in the way driving an early, expiremental car is probably hard.
Trying to read Joyce for entertainment is like trying to learn physics from "Principia." Cute, but probably a terrible idea. Reading either to engage with the history of novels or physics... this is why people still read these today.
People might get a sense of satisfaction frim the difficulty, but this does not mean we need people to write like Joyce or Newton.
There needs to be an honest reason for difficulty, and the problem with most difficult writing is bad reasons. Poor technique. Partially formed thoughts. Intentional obscurity. Orwellian use of jargonized dialect...
Finnegan's wake would have been a better work if it was written easier. The difficulty is a flaw.