> Maybe it's a rural Texas thing?

Not really. Maybe for the author. Living a long time in rural Texas (now urban), I can say that someone might have a propensity to use over and out for specific directions, but in my experience they are interchangeable and it's not a universal truth. Granted there are no universal truths about Texas/Texans so when you hear one, discount it. And you are right that up and down as cardinal directions doesn't feel uniquely Texan but can't tell when only on the inside.

In the seemingly endless rush to satisfy Texan exceptionalism, dialect and slang are among the traits that are always held up as unique. Yet every region has their own.