they threw it on all the millenial math problems (I think there are 6 at this point unsolved, well, 5 now).

And according to them at some point they saw that one was close to being solved, so they pointed all the agents at it.

The same thing happens to humans - at this time there are no simple problems left, so solving the hard ones requires using prior knowledge and attempts at solving things.

Yeah but the one they decided the AI was close to solving may have been so because the researchers' progress on this problem became part of the training data for that AI...