The difference here is that Nim compiles to C and you can turn the garbage collector off: Zig compiles C and there's no garbage collector. That means the entire standard library is available when generating object code. It's also trivial to opt-in to the C ABI on a fine-grained basis, by defining a function or struct with the extern keyword.

I believe this is still fairly current about the difficulties of building Nim dylibs for C programs: https://peterme.net/dynamic-libraries-in-nim.html

I expect Nim will stabilize about where D has: it will have a dialect of the language which, with relatively painless accommodations, is able to produce object code which speaks C ABI. Zig is different. The language is relentlessly focused on providing a better alternative to C while occupying the same niche, and a lot of design time has been spent on making it practical to take an existing C program and start writing the new parts of it in Zig.

It's a good language, Nim, and getting better. I'd recommend it for someone who is considering Go, for example.