alt Hacker NewsI also am skeptical of the "LLM output is derivative of everything in the training corpus" argument in general, but in this specific case I think it may have more merit. If the model was trained on unrar source code, and obtained specific information about the RAR format from that code which it then used in the code generation step, then the output is arguably tainted because of that.
Does the source-available UnRAR do anything that the existing FOSS implementations can't do? IIUC the interesting part of this particular project is that it supports really old versions of the file format that were never publicly documented anywhere.