Scott Ainslie in his book "Robert Johnson" (transcription of 28 Robert Johnson tunes) states:

During the middle to late 19th century, the term 'California' bcame synonymous with the gold rush, wealth, and money. Many a prospector rashly abandoned their homes in the kEast to go to California, where the gold was supposed to be just lying on the ground waiting for some lucky person to pick it up. Here Johnson seems to be inviting his woman to accompany him to a land of golden opportunity -- which was not California -- but for a blues musician in the 1930s, definitely was Chicago.

This makes great sense to me. I do not think Robert Johnson, who travelled quite a bit through the Delta, was that ignorant of geography.

Roger in Baltimore