If you can formulate your problem in the sense of classical planning problem, there are different sets of planners that can solve the problem. These methods are explainable. Some of them even give the optimal solution to your problem if you use optimal heuristics. This is what he means by that statement. The only issue is, right now the classical planning is hyperfocused into few problems and most of the methods work on narrow domains like path finding, logistics, constraints based problems and so on.