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On the other hand the author is pretty explicit that this is a limited borrow from Clojure, and that when in conflict, he’ll do the more Rackety thing.

Yep. I'd say persistent collections and (lazy) sequences. And great concurrency stuff (agents, atoms etc.)

This. Without persistent collections, any lisp that looks like Clojure is only half way there.

This got me curious about whether persistent data structures are available in Racket. It seems like they are.

https://docs.racket-lang.org/functional-data-structures/inde...

Being fundamental to clojure means everyone uses those data structures. That matters quite a lot when you aren't writing everything from scratch.

There is also immutable hash-tables available: http://docs.racket-lang.org/reference/hashtables.html

(Just to be clear: there is of course also mutable hash tables available).