"Fearless Concurrency" predates "async" in rust nomenclature, and points to specific compile-time guarantees around things like thread safety and races (Send, Sync, ...).

It was around when we used event loops and the mio crate (and rayon for parallelism), with no async or await in sight (because they didn't exist yet).

Some people actually still prefer to do "concurrency" this way, especially if they wish to stay close to the abstraction level from C land. io_uring also came later which introduced a new OS low-level interface and paradigm for doing "concurrency".

"Fearless Concurrency" has nothing to do with how easy you can write async code, especially if you're someone who struggles with the language basics like ownership.