I'm not sure that's true. Much of the imagery can be interpreted allegorically - not just concerning the obvious themes of grief and coming to terms with the loss of his mother (and accepting his stepmother) but things like the parakeets potentially representing Imperial Japan. Plus the unspeakable power held by the wizard could potentially pertain to nuclear weapons, though on the other hand the "build your own tower" message he delivers could also speak of breaking free from the Imperialist mindset. It's a fascinating film, not to mention stunningly beautiful. I loved it, though I don't think it is quite up there with Spirited Away as Miyazaki's greatest.

This is my review, if you're interested: https://medium.com/simon-dillon-cinema/film-review-the-boy-and-the-heron-5c667031a81f