Go handles concurrency via goroutines whereas in Zig... I couldn't find any available information. All it says in their docs "it's an area of active research".
Zig has stackless coroutines, I wrote a blog post that explains how it all works at a high level:
I don't see how this follows. Async/await is a disaster in every way, M:N threading (which Goroutines are an implementation of) is the way to go. Not only does one get sane concurrency but also parallelism. Look at Ada tasks and Erlang processes for writing parallel code with critical requirements of correctness and efficiency.
This is not something that a systems programming language would want to do. I agree with your point that structured concurrency is good and preferable for high-level languages, but it's not an absolute better choice when you start thinking about the details, like you have to do in a systems programming language.