If I read it correctly, California state constitution amendments are from popular vote in the state - the California legislature can only put them on the ballot to be voted on (and then, its a 2/3 vote there - easier to get 8% of the previous total vote to petition).
The Supreme Court can't rule against it because it is constitutional because it is the constitution.
> In 1992, a legal challenge (Nordlinger v. Hahn) was considered by the United States Supreme Court, which subsequently ruled 8–1 that Proposition 13 was constitutional.
The Supreme Court can't rule against it because it is constitutional because it is the constitution.