Sure, although in this sense C also has a runtime. I assure you there's no magic where CPUs just know that the C main() function is where your program starts. That's what CRT stands for if you see things like "crt0.s" or "crtmain" mentioned, C Run Time.
This Rust code is handing over from your operating system to the main() function of your Rust program, it's arranging that you've got all those modern application amenities like command line parameters, environment variables, it will name your thread (if the OS has names for threads) and so on.
Obviously if your Rust is firmware for $5 device it probably doesn't have an operating system, and it certainly doesn't have command line options, accordingly no_std (the Rust environment you are writing for) doesn't do this stuff. You will wake up alone, with the function you annotated (IIRC with #[start]) running and only the features from Rust's core library. The equivalent in C is standalone mode, C doesn't give you a library at all, just your language, operators etc and your wits. You will of course write your own library unless your project is tiny, because this is pretty unsatisfactory.
However, Arti is not intended to be firmware for a cheap piece of electronics, so both the C Tor implementation and Arti will end up with this very thin runtime to run as applications, the runtime just sets things in motion and calls your main function.