The next paragraph they state:
We looked at and compared several languages - Go, Java, C++, and Rust. We knew that C++ was harder to scale and maintain high quality as you build a dev team; that Java doesn’t provide the flexibility and systems programming language we needed; and that Go is also a garbage collected language. This left us with Rust. With Rust, the pros around performance, memory management, and ease of use outweighed the cons of it not yet being a very established language.
The article is a short recap of a 40 minute video. The video has more context and explains the intentions much better than the web page.
They show a graph of performance over time as the rewrite progressed. There were some small optimisations and problems, a few big regressions, and then a huge improvement that was maintained. Looks like the rewrite process made the database perform significantly better. There's nothing on how much this was caused by the language switch itself, but that's functionally impossible: nobody is rewriting their application twice to see what rewrite is better.