It's not hard to know - vibe-coding is the default mode for all LLM coding so one can just assume as much absent evidence to the contrary.
The blogpost about the compiler is AI written, the README file in the Github repo is AI written, all the documentation is AI written, the reverse engineering analysis of Borland Turbo C++ is AI written, all the comments in the code are AI written, and every single commit in the repo was done by Claude. I don't think it's reasonable to say that the author took any serious interest in the code.
It's easily to spot a code base that's build LLM-assisted. But to know that no human actually took interest in the code is hard to know without talking to the humans that prompted the LLM.