Has to be something other than Rust to be able to be "rewritten in Rust"

Their inference server/service (wrapper around this engine) could still be written in Rust, this is just the engine.

This is written in Rust: https://github.com/deepseek-ai/deepseek-recipe

This DeepSeek Recipe is also mentioned here: https://zhuanlan.zhihu.com/p/2095560295930508953

Given how much the models like React and JavaScript I guess we should be thankful its Python.