This is a legitimate way of viewing things. I wonder how “true” it is in the sense that explains Mexican food in California today. Specifically, I don’t think the Mexican population in Alta California was really all that high in 1848. I think it just wasn’t all that high in general.
So there are families that can trace themselves to Mexican California, but I believe their numbers and culinary influence was overshadowed by the significantly bigger population that came later. But maybe that’s just as authentic. They were bringing and making dishes, not copying them.