This does not mean that changing from tabs to spaces will increase ones income. I would expect there simply is a confounding factor that the author did not look at. Maybe the info is not in the data. I can imagine that the space/tab choice is related to the "upbringing" of the developer. Maybe which language or editor they used first in their life. Or maybe it's related to culture. For example when using IRC, tabs ar…
If you are not born in the nobility that uses spaces, but if at first contact the people in power see you using spaces they might assume you are born a noble, and lower their anti-commoner guard for you and you have a greater chance of entering the nobility without a proper title (don't forget to slam the door after you, nobility only works on exclusivity).
So there is a path for taking the implication completely backwards and still reaching the expected outcome.