It is probably fine, it is kind of a best case scenario: porting a good code base with lots of unit tests, all hand-written. Not much can go wrong here as the LLM is kept in check by the original code, the tests, and the fact that the topic (a JS engine) is well documented.

The problem is what comes next. They now have code that they don't understand, and they are likely to work on it with AI in the future, but the new features they may introduce later will not have the luxury of hand-written tests and a reference code. So, unless they undertake the massive effort needed to fully understand the Rust code and deal with all these "unsafe", quality is very likely to go down, Microslop style.