It's not like C++ only focuses on features with zero overhead. The zero-overhead principle means that you don't pay for what you don't use. It doesn't mean that compiler developers are forbidden from working on any language or library feature that consumes more than zero cycles at runtime.
I think the terminology here is looser than it should be, and these are not really the same thing, even though I've seen zero-cost abstractions used to describe them both. It causes confusion sometimes, as I believe it is doing here.
[1] https://en.cppreference.com/w/cpp/memory/gc/declare_reachabl...
[2] https://herbsutter.com/2011/10/25/garbage-collection-synopsi...
[3] https://web.archive.org/web/20120228143039/http://www2.resea...
Rust has `Rc`, which is not atomic but `!Send` so the type system ensures that it stays within a thread, as well as the atomic `Arc`.
You can see it here on HN. What's the #1 comment that always appears on Rust async threads? Invariably it's "why not just use goroutines? That kind of code is so much easier to write!" Now for various reasons M:N threading like in Go doesn't work in Rust. What does work, and offers most of the ergonomics of threads/goroutines, is async/await. That's why the team has prioritized it.
There are people who care less about ergonomics than optimizing every CPU cycle for socket servers. For those people, the focus on getting the ergonomics right before microoptimization of mio and tokio might be frustrating to see. But the overwhelming majority of users just want easy-to-use async I/O. Async/await is an enormous improvement in readability and maintainability over hand-written state machines. Rust would be doing a disservice to its users if it focused on microoptimization of tokio instead.