I've always assumed it to be the nation as in Woody Guthrie;
Way don yonder in the Indian Nation
Ride my pony on the Reservation
"No one in Texas or the US would have considered using the Indian Nation in those days."
Texas was a different place before the swarm of Southern Whites changed the ethnic balance.
Remember the "Yellow Rose?" She was the "sweetest Rose OF COLOR Texas ever knew."
It doesn't mean he was Amerindian. Lots of reasons to end up in the Nations at the time. Some good, some bad. I'm especially proud of an ancestor of mine who, after taking part in the military escort to take the Choctaw from Mississippi to Oklahoma, resigned his commision, married a Choctaw girl and settled down.