It sounds like you didn't read the blog post. The whole point is that they are not rewriting code, but writing new code in memory-safe languages (including "niche" languages like Java). This strategy is paying off with fewer severe vulnerabilities due to memory safety bugs.
where did you get the impression they are not rewriting code in rust? because the author cited some other blog post saying they should focus on new code and not rewriting it? How does that translate to "The whole point is that they are not rewriting code," ???
Obviously they're going to be rewriting whatever they see fit. If YOU actually read the article you would have seen they have "keystore2" listed as new rust code. I WONDER what was keystore1 implemented in? Nice try, but no cookie.
Yes, one is named "keystore2" but you don't seem to actually know anything about it (nor do I). It seems doubtful to me that they would rewrite a perfectly working component in Rust just for fun. Most likely the original keystore, whatever language it was written in, was not meeting its requirements and needed to be replaced.