I have a hunch that rusts global compilation model (as opposed to C++ separate compilation, as crippled as it is in practice due to header-only libs and link times) should always lead to C++ scaling better with the size of the project.

Simply put: Rust doesn't offer an abstraction past type checking. Rust could potentially introduce its equivalent of PCH, or maybe it already does so in some internal cache, but as long as it uses monomorphization and no PhD student comes up with a clever trick to combine that with separate compilation it will scale worse.

That being said, maybe a more fair comparison wouldn't involve polymorphism in the rust code? C++ doesn't really have polymorphism, it uses templates for a similar feature. So what happens if someone replaces rusts polymorphic functions with macros?

This is pure speculation, but I suspect egregious use of proc macros in Rust also inflate compilation times a bit. Just speaking anecdotally, but removing proc macros in one of my hobby projects decreased clean build by about ten seconds. Definitely not a couple magnitudes like the author saw, but not unnoticeable either.

I always wonder: If the proc macros were executed JIT-like with the Cranelift backend instead of compiling them with the LLVM backend, wouldn't that drastically reduce build times of proc-macro-using code? The Rust compiler spends most of its time in LLVM, if I recall correctly.