Nix builds binaries. You don't need nix to run binaries built using nix. Just like you don't need Cargo to run binaries built using cargo. This is about developer tooling.
We should generally be pushing in the direction of developer tooling being available to more developers on more platforms. One of Rust's great strengths is good Windows support. Not supporting native Windows makes Nix a non-starter in many of these discussions.
It's interesting how many people comment on nix without seeming to know anything about it. Why?
Nix doesn't replace cargo. It would use cargo under the hood. If Cargo supports Windows builds, those builds will work with nix. However, nix (probably) doesn't give you anything additional in this case.
I'm not sure what you mean then. Clearly you have to put a bunch of effort into building tooling that works on Windows, and Nix won't cut it for that (you must build on Windows in most cases rather than cross-compiling). And if you're doing that you might as well also reuse the same tooling on other platforms.
Nix flakes can still be useful on the tool user side, if your application build requires Unix anyway, but on the tool producer side it's a bit less so.
Nix normally binary edits /nix/store paths all over the place in the usual built binaries, those won't exist without nix installed & dependencies downloaded.