In my experience, short identifiers in Clojure tend to be pretty standard things, e.g., f for a function, coll for a collection. Certainly this article has some pretty cryptic examples, but the author agrees with you and me and ends up fixing them.
No it's not. There's no style-guide rule, or PEPx, or core team recommendation to use short or long names. Clojurists are pragmatists to the core — they use whatever makes sense in the context. If `m` and `f` feels like a reasonable choice instead of `map` and `function` — that's how they'd name the vars, but it's not a universal rule.
Clojure's syntax (and Lisp's in general) is very flexible, you may encounter names like `bar->foo` — for transforming, or `bar+foo` — for data combining functions. For uninitiated these kind of names may look cryptic — when you encounter a function name like that for the first time, you'd think it's some kind of special language construct. Matter of fact, because Lisp syntax is so minimal, the overhead of learning mental parsing of the code is also (relatively) small. No one after a few months of writing Clojure conceives it as a atypically "cryptic language", but of course, there are programmers capable of writing cryptic code in any language, no matter how generally terse or verbose it is.