That's only vis-a-vis the Main.java Java implementation, which has a different algorithm, not implementation. The author is not referring to the CL implementation (note that the author explicitly calls out Main2.java, CL, and the Rust versions as being the most similar and worthy of being benchmarked). The Main.java one is more of a curio than anything else, which the author threw in because it was the first implementation they wrote (the article acknowledges it has a different algo and is therefore not directly comparable to the other two). This is not what the author is referring to when they say "well-optimised" (note that the article explicitly excludes Main.java from a lot of its valid comparisons, e.g. "Because the Rust code implements a similar algorihm to Java’s Main2, NOT Main, we should not conclude that Java can beat Rust in speed!")

The main comparison of the article is Main2.java, main.rs, and main.lisp, as the author both calls out in the article and the attached GitHub repo (as is apparent in the author's choice of optimizations; if it was a comparison of algos, then the CL and Rust versions would be rewritten to use tries as well).

The point is the author did not explicitly call out this current Rust iteration as explicitly suboptimal RE CL. The closest the article comes to calling the Rust iteration suboptimal is

> However, the Java and Rust implementations were, as CL’s, written without much thought given to performance, after all the description of the original study which introduced the problem asked participants to focus on correctness and readability, not performance.

which is referring to the previous iteration of the code, not the current one.

(It's also evident from Cryptonic and chrismorgan's comments they are talking about implementation-level concerns, not algo-level ones such as Main2.java vs the other implementations)