Yeah, I think it is very obvious that rewriting something carefully in Rust for your specific use cases is going to be more robust, faster, and more featureful than something general purpose. You could probably cut 95% of the lines in Nginx to meet your use cases after all...
The thing I'm interested in from the article is what they did to replace their lua runtime. It was used for request processing and WAF rules. Is it now baked directly into Rust? If a new WAF rule comes out do they have to recompile their entire proxy? The dynamic nature of Lua and Nginx was very powerful - I wonder how they changed that or what they replaced it with.