They most likely refer to homoiconicity [1], as Clojure is a dialect of Lisp. However, it's hard to say for sure, and maybe they were simply referring to the built-in syntax for maps, lists, etc.
Not only due to homoiconic nature. All (well, technically not all, let's say most) Lisp dialects are homoiconic. Yet, there are some other aspects that make Clojure specifically well-suited for data manipulation:
- immutability and persistent data structures (makes code easier to reason about [the data]; enables efficient concurrency - no locks; some algorithmic tricks that makes it very performant despite having to create copies of collections),
- seq abstraction - unlike other Lisp where sequence functions are often specialized for different types, Clojure simplifies things by making baked-in abstraction central to the language - all core functions work with seqs by default. it emphasizes lazy sequences as a unified way to process data, i.e., memory efficiency and infinite sequences, etc.
- rich standard library of functions for data transformation
- destructuring - makes code both cleaner and more declarative
- emphasis on pure functions working on simple data structures
The combination of these features makes data processing in Clojure particularly elegant and efficient.
[1]: https://en.wikipedia.org/wiki/Homoiconicity