Your answer doesn't answer the question. Where does the book say that wizard and cleric's spells are the same in the fiction, as opposed to using the same mechanics as a useful short hand?
I didn't say the spells are exactly the same. I said the prime source and interface of spells are the same.
A cleric1/druid1/wizard1/sorcerer1 spell slots merge now into a 4th level spellcaster.