The article mentions that the Bootstrappable Builds folks don't allow pre-generated code in their processes, they always have to build or bootstrap it from the real source.

Because that wouldn't be reasonable to audit. The program that compiles the new Rust compiler, as well as the programs that disassemble and decompile, could insert backdoors or other nefarious behavior into the generated C, in a way that could be difficult to detect.

The "ethos" (for lack of a better word) of these bootstrapping projects requires that everything be written by hand, and in a way that can be auditable.

Wait, dissemblers will turn assembly into any language you want?