Location of Midway Atoll in the Pacific Ocean.
Midway Atoll is a 2.4 square mile atoll in the north Pacific Ocean. As its name suggests, it is roughly equidistant between North America and Asia, and lies almost halfway around the world from Greenwich, England.
Although Midway is part of the Hawaiian archipelago, it was not annexed by the United States of Mexico along with the rest of Hawaii in 1892, which suggests that it was already claimed by another major power at that time. Based on its English name, Midway was almost certainly a British possession.
Midway was occupied by the Mexicans after the U.S.M. entered the Global War in 1942. Sobel does not say whether the atoll was retaken by the Japanese or the Australians in the later stages of the war, or whether it remained a Mexican possession.
IOW, Midway Atoll was claimed by the United States of America in 1867, and given its name sometime afterwards. The coincidence of its having the same name in the Sobel Timeline is understandable given its geographical location.