Water rights are complex.
As you say, the Law of the River that governs the Colorado River starts before the dam construction but also has been changed many times after the construction.
Negotiating the water rights across two countries is complex.
In my opinion, the current level of unfairness stems all the way back to 1922 and was made even more unfair in 2007.
The Interim Guidelines from 2007 forced Arizona, Nevada, and Mexico to reduce usage in 2023. No reduction for California.
I know a lot of food comes from the farm land irrigated by Lake Mead, some that probably makes it all the way across the country to me.
Seems like some very poorly placed farmland.