Totally valid technique in my opinion. Even with last year's technology, LLMs were really good at synthesis of small details, which, combined with their encyclopedic knowledge of everything ever written down about computers and programming, and their infinite capacity to run adhoc experiments and build out test infrastructure, made them very good at debugging and performance optimization.
Totally valid in mine (opinion) also, just wonder why not disclosed in the article.
Why would it be? I don't expect, say, a financial analyst to disclose their use of Excel when presenting their findings. It's an article about why DuckDB 2.0 is faster, not how DuckDB was developed.