But "learning a new language" can mean multiple different things. It can mean learning a language at a superficial level, but it can also mean learning the depths of the language, or the depths of the ecosystem, or both. For instance, for Java, how many professional Java developers understand and know about memory consistency and the Java concurrency model? Or some other niche topics that might be important in some contexts? How much time would it take for someone to learn the ins and outs of unsafe Rust or pinning in Rust, or other involved topics? Or Rust's gotchas?

There are different depth levels of learning a language. And there are lots and lots of topics besides the specifics of various programming languages or their ecosystems.

I barely knew Rust, yet I was able to rather easily dive into and fix significant bugs in a Rust project that the experienced Rust developers on it had struggled with, possibly struggled for months. But the cross-discipline set that was relevant for solving those specific bugs was also a good fit for my competencies as well, and I have lots of experience with OOP, FP, systems programming, etc. FP is not necessarily learned in a day.