So someone who wrote their own functional operating system and browser from scratch which he is now targeting the public with, is not comfortable learning something new?
You are all assuming that the project will be c++ only when the authors haven't said anything about the matter. Who knows if they aren't open to moving to rust? The project is originally in c++, not only but, because that's what the target OS supported. There are examples of other browser moving from c++ to rust (Firefox) who says they can't do the same?
I bought CLion's license for many years for personal use. I could easily work on c++ and python on the same project, and could still use it for Rust (same project or not). I decided to stop with the license when they deprecated Rust's plugin in favor of RustRover. I don't like jumping around between "different" IDEs.