The article later says (about the server/client example)

> Unfortunately this code doesn’t express this requirement [of concurrency], which is why I called it a programming error

I gather that this is a quirk of the way async works in zig, because it would be correct in all the async runtimes I'm familiar with (e.g. python, js, golang).

My existing mental model is that "async" is just a syntactic tool to express concurrent programs. I think I'll have to learn more about how async works in zig.