David Parker is an idiot. It’s common knowledge that Baum was a follower of Theosophy, a pseudo-philosophy that was very popular at the end of the 19th century. All the symbols in the Oz books - he wrote 14 - were straight out of Blavatsky’s writings.
Baum himself said it was just a story in fact in later editions he added that to the beginning of the book hence my quote in the article. Sometimes we don’t need to read deeper. Just because Universities professors especially those in English department tell us there is something there doesn’t mean there is.
One of the things that should be noted is how the movie worked to dilute all the metaphors in the book(s).
For instance the whole silver/gold standard thing can’t be seen unless one knows that Dorothy’s slippers were silver in the book and changed to ruby in the movie because they looked better.
The metaphors in Wizard are legion and consistant. It doesn’t so much require one to go looking for them as realize they’re being beaten over the head with them.