In particular, the Supreme Court held that in 1984 that this provision of the California constitution can't override the scope of property right that conveyed from Mexico/Spain after the Meixcan-American war: https://scholar.google.com/scholar_case?case=122714134443780....
The California court of appeals has already applied that precedent to this case. That was resolved years ago.
Edit: The president can unilaterally withdraw from a treaty, and Congress can certainly repeal a treaty (or even edit it) using the typical lawmaking process. But passing a simple law isn't going to override a treaty on it's own. And it would almost certainly have international repercussions.
That said, in this case, the rights have already been granted to the property owners by the US in the form of a converted land patent, and changing that retroactively would almost certainly count as an expropriation / taking anyway.
So you might as well expropriate from the get go, without going to the trouble of dealing with Congress.
And they can do so with or without consulting with a treaty partner. It's called sovereignty.
And in the US, the supremacy clause of the Constitution (Article VI, Clause 2) states that ratified treaties are the "supreme law of the land," second only to the Constitution itself.
The Supreme Court, in it's famous Holland ruling, clarified that this includes laws and policy that otherwise would be a State matter, like land rights.