It seems to me it would have been highly likely to be more expensive and more resource intensive - if realistically possible at all, short of implementing a general Zig to Rust translator first.
A recent and highly relevant example is the migration of the TypeScript compiler to Go. They did not use an LLM to translate the code. Instead, they used LLM assistance to write a deterministic TypeScript-to-Go translator and then used that to translate the code. I have far more confidence in this approach than in letting the LLMs rip on the translation itself.
Leaving aside ownership, Rust is a big, complex, expressive language. I'm not that familiar with Zig, but I think it tries to be a "better, modern C" so it seems like it should be easily possible to mechanically translate Zig into direct Rust equivalences. You probably won't get "good" idiomatic Rust at the end, but you should get working code that does the same thing.
I've done Perl to Java, Java to Kotlin, Python to Ruby, Ruby to Java, C to Swift, you name it.
It's only when you change behavior during the rewrite that it becomes an intractable problem. If you ship a 1:1 translation, THEN you can start going through the list of "bugs" you found along the way. Tread carefully when it comes to this, however, as I can almost guarantee that within your non-trivial codebase there will be some code that implicitly _depends_ on a "bug" to function at all. This where shit hits the fan.
if (PREFER_EXISTING_BUGS_TO_NEW_ONES) { theOldBehavior(); }
else { theNewBehavior(); }