Take the computer which beats Magnus and restrain it to never make the best move in a position. Expand this to N best moves as needed to reach 1300 rating.
Despite decades of research, nobody has found a good way to make computers play like humans.
This means that the LLM does not actually have a lot of training data available to learn how a 1300 would play, and subsequently does a poor job at imitating it. There is a bunch of papers available online if you want more info.
I think the chess model would be successful at producing the desired outcome but it's not as interesting. There's something to be said for being able to write down in precise terms how to play imperfectly in a manner that feels like a single cohesive intelligence strategizing against you.