From what I can see looking at the treaty, and looking at the Supreme Court case, it seems to me that you are right. I do not see how either of them prohibit what California wants to do, provided that California is not singling out Mexicans. It's also not clear to me from the treaty if it even applies to people who were not Mexicans at the time the treaty was enacted.

Note: I have not extensively researched this. I googled for the text of the treaty, and then did some searching within it to try to find the parts relevant to property rights. I also googled for the Supreme Court case and read the decision (which was short and not very informative by today's standards). In other words, I could be completely off on this.