Servo was on a multi-year development hiatus, which is the reason this is news (see the graph in the article). In addition the original priority of Servo was not to broadly implement web standards, it was as a proving ground for Rust components to be uplifted into Firefox (I'm unclear whether or not those components, which have surely been continually developed since then, have been backported into Servo).
Whisper it but C++ is probably a better language for building a FOSS web browser engine than Rust.
There’s simply more C++ programmers around, and you need as many bodies as possible for such a large project. There’s also precious few Rust developers with experience with large projects since Servo is the largest project.