What seems bizarre though is that the language problem was fully solved first (where fully solved means AI can learn it through pure observation with no human intervention at all).

As in language today is learnt by basically throwing raw data at an LLM. Board games such as chess still require a human to manually build a world model for the state space search to work on. They are indeed totally different problems but it's still shocking to me which one was fully solved first.

>Board games such as chess still require a human to manually build a world model for the state space search to work on

That's not so. Deepmind MuZero can learn most board games without even being told the rules.