I'm not sure how moving from JS to an systems engineering language would make anyone more productive. I bet they just wanted to use Rust, because why not?
If you read the post, it’s because they’d made the decision not to use any third party dependencies in JS due to perceived problems with quality and correctness, and they found Rust crates to not suffer as much from these problems. So their productivity mostly stems from not having to rewrite their dependencies anymore, not so much just the switch to Rust itself but the whole ecosystem.