I meant that for AI we will possibly require high-level credit assignment, e.g. experiences of regret like "I should be more careful in these kinds of situations", or the realization that one particular strategy out of the entire sequence of moves worked out really nicely. Instead it penalizes/enforces all moves of one game equally, which is potentially a much slower learning process. It turns out playing Go can be solved without much structure for the credit assignment processes, hence I said the problem is non-existent, i.e. there wasn't even need to consider it and further our understanding of RL techniques.