From MSN Encarta: "When Mexico gained its independence from Spain in 1821 it assumed ownership of western and southern Colorado. In order to secure the frontier, the Mexican government awarded large amounts of land to Mexican citizens who were willing to establish colonies in the San Luis Valley and other border areas, but few settlers moved there. Mexico was forced to cede its territories in what later became the southern part of the United States, including Colorado, to the United States following the end of the Mexican War in 1848. The U.S. government recognized the original Mexican land grants, and colonists, mostly Spanish and Mexican, began to settle in the San Luis Valley during the 1850s."
Thanks for your comment about Mexico using US settlers as a barrier to the Comanches. Never heard that before, but it makes a lot of sense.
Note that the quote you cited merely said that the Mexican government ASSUMED its ownership and made large land grants. Then Mexican citizens and Spaniard began settling the San Luis valley in the 1850s AFTER the Mexican War settled the claims of the Mexicans to territory CLAIMED by the Spanish.
It was the security that began to be provided in those territories by US settlers and military that enabled the Mexicans and Spaniards to begin to settle their land grants.Because the US is a government of law, they mostly honored the land grant claims.
Outlandish claims of soverignity over lands now part of the US were made by Spain and assumed by Mexico without actually exercising any control over those territories.
Remember that Spain and Portugal got the Pope to divide the world between them after the discovery of the New World. Doesn't mean it happened.