The word simple is used here a way I'm having trouble wrapping my head around.

This specific usage appears to come from this linked talk, Simple Made Easy:

https://www.youtube.com/watch?v=SxdOUGdseq4

My reaction to the Unix pipeline was that, the reason it exploded in complexity is because the pieces were too simple. They were insufficiently expressive.

But the word is used in a different way here, and I'll have to watch the talk to understand what exactly is meant. (Something like orthogonality?)

I don’t know. This reads more like a “Clojure is great” post, and Clojure is great. But the author takes a swing and a miss on the Rich Hickey magic. The UNIX example is contrived (the number of occurrences in the order they occur?), and trying to redefine simple in a way that excludes UNIX pipelines doesn’t work.

Agreed. There are multiple senses for "simple" and on of those senses is "small". It is actually a very useful sense too, as Solomonov/Levin/Kolmogorov/Chaitin-style inductive reasoning has shown.

And it is important not to just make snappy quips by equivocating.