but it is still useful for explaining why planning is non-trivial.
...when thought of in the wrong way.
However, noninterleaved planners typically separate the goal (stack A atop B atop C) into subgoals
That's already wrong because these subgoals
are not independent! The structure of the problem is not independent. The blocks must grow from the bottom. The first subgoal should be "get C on the bottom". This is not "tower of Hanoi" and not all problems are Hanois.
A good example of why that "break things into little pieces" is NOT always going to solve the problem or solve it efficiently - the whole needs to be considered, not the parts.