The claim was that scala code is very difficult to read and idiomatic scala code, as evidenced by their collections library, brings along a ton of conceptual overhead that creates much more complexity than it eliminates.

That line alone drives home the point for me, you can read the rest of scala.collection.immutable if you need more convincing.

The claim was that you need to know category theory. Your post is still there, we can still read it. It does not make much sense to pretend it says something other than what it says.

First and foremost, I think you need to read about Blub. [0]

Also, idiomatic Scala code is perfectly readable. You have no right to complain about readability until you've dealt with spaghetti code written for Megacorp Inc, that completely ignores the fundamentals of structured programming. :)

[0] http://www.paulgraham.com/avg.html