In my experience Rust development is no slower than C development (in a different environment) or C++ development (in a comparable project)

I think they were using "writing Rust" in the most strict sense: the part of the development cycle that involves typing the majority of the code, before you really start debugging in earnest and really make things work.

But their point is that "developing Rust" (as in, the entire process) ends up being a similar total effort to C, only with more up front "writing" and less work on the debugging phase.