The demoscene thinking is - I've been led to believe - that given most of the coding is done on assembly level, anyone with the binary already has the code.

Apart from old-school productions, there's very little assembly in modern demoscene. 64kB intros use mostly C++ (or Rust). Even for 4kB intros, assembly is not used very often (and the interesting code is in the shaders anyway).

Who's writing demos in rust?

Logicoma's demos are almost entirely written in Rust, last I heard they were only using C++ for their audio synthesizer. Examples:

http://www.pouet.net/prod.php?which=69658 / http://www.pouet.net/prod.php?which=68375

Their coder Ferris answered some questions about the implementation here:

https://www.reddit.com/r/rust/comments/597hhv/logicoma_elysi...

Surely the assembly code has comments, names, etc. The code might be closer to the binary in most programming languages, but it isn't the same.

It depends what year we are talking. C64 was mostly assembly, farbrausch eas cpp iirc.