You are right, it is very far off from idiomatic Rust. It could have been done in virtually any programming languages. I am not sure why Rust was chosen if they decided to go against everything Rust stands for.
This is how a lot of rewrite projects start out, particularly in the hobby space. They focus on a 1:1 translation between the two languages until they're able to drop the original language's dependencies (compiler, etc). Then from there you start translating into idiomatic rust.
I have never seen rewrite projects start out this way. What is the point of the rewrite then, if this is 1:1? You will definitely implement bugs, too, and most likely introduce more.
Any ship of theseus rewrite is going to introduce bugs in the process, either by translating it 1:1 first or by replacing code with rust and talking over FFI.
The appeal of translating to rust first is that 1. you intimately learn a codebase you did not write, 2. you can refactor code during the following stages of the rewrite without worrying about FFI boundaries (which are their own massive source of complexity), and 3. you can work entirely in an ecosystem you prefer rather than attempting to work with tooling across ecosystems.
I personally prefer this 1:1 approach for hobby projects (where notably you don't have to continue maintaining a working product while the rewrite is in progress) because you can refactor in passes rather than in sections but it does have it's own drawbacks (added bugs/changed behavior from translation, etc) that you have to compensate for with additional tooling (like fuzz testing, conformance testing, etc).
If you still think they are using a transpiler rather than doing rote hand translation, please point out some specifics.