If the US promised, in the 1848 Treaty of Guadalupe Hidalgo, to honor in perpetuity the property grants of Mexico in the conquered/ceded territories, maybe that promise still applies. And if the Supreme Court already ruled (in 1859!) that California law can't alter those rights, just as state law is limited in other international and interstate affairs... well, it's an interesting case. Would love to see Volokh Conspiracy or Popehat discuss it.
Khosla may yet want to cede this claim, even if it's legally legitimate, to not appear like a robber baron buying up eccentric anti-democratic privileges that were created by ancient wars.
Update: And another thought... maybe if the US and California now view the coast – and the water-rights angle that ChuckMcM mentions – as requiring a new approach, they can negotiate a treaty-patch with Mexico? :)