Might Spirited Away be a movie about a little girl growing up through her experiences in a fantasy world?

It is. Miyazaki himself said that he was mostly thinking of the many capable kids and teens he knows, who are really good people but very afraid of the big world awaiting them when they leave home and start working. He wanted to communicate their fears, but especially give them hope. He also went very strongly by the narrative structure of the monomyth, so there's another non-Marxist bit.

You're definitly right that much of what's listed there are also common elements in children's story in general, emphasising common values such as teamwork and caring about people more than money.

But some of these moments are just so extremely telltale for a Marxist perspective. In particular that scene that Zeniba's hut and this noteworthy emphasis on mutual work. It's a moment that has no relevance to the plot, yet somehow made it in there.

If it was another writer or director, I would also agree that it might as well be a coincidence. But considering Miyazaki's particular background as an active communist, it's just too fitting. He wouldn't miss that connection.