I think the author's referring to the multiple implementations of persistent maps (i.e. APersistentMap). Clojure's generated docs[1] say there are four implementations (PersistentArrayMap, PersistentHashMap, PersistentStructMap, PersistentTreeMap), and this Stack Overflow answer[2] explains that the latter two are rarely used, and the former two make up the bulk of actual APersistentMap instances, with the language's core functions for persistent maps changing implementation used under the hood as it likes in order to do what it thinks is best for performance (basically, using PersistentArrayMap if the number of keys is small, and using PersistentHashMap otherwise). You can see this, for example, in PersistentArrayMap's implementation of assoc[3], which replaces itself with a PersistentHashMap if the number of keys has grown over the hardcoded threshold of 16.

So like you say, your experience as an "end user" makes it seem like it's just one data structure at least in interface, though in implementation it's actually doing some performance optimization stuff which, I think I'm willing to grudgingly grant to the author, does make performance harder to reason about in principle. Though in my (very unserious) experience, I think I've enjoyed languages that take a possibly incomplete stab at getting performance right over ones that draw a line leaving it entirely out of scope and declare victory.

I'll equivocate a little and share that I have been bitten by these internals before, though the fault was mine: one time, I was unwittingly relying on an implementation detail of PersistentArrayMaps, which is that they preserve insertion order. PersistentHashMaps don't, and my code was mysteriously breaking when number of keys grew beyond 16 until several hours later I learned much of what I've just shared above.

[1]: https://www.javadoc.io/static/org.clojure/clojure/1.10.1/clo...

[2]: https://stackoverflow.com/questions/16516176/what-is-the-dif...

[3]: https://github.com/clojure/clojure/blob/5ffe3833508495ca7c63...