I'm no expert, so maybe Pascal or someone who knows what I am talking about can chime in to correct and elaborate.
Basically he is talking about the Blues. The evolution of the Blues has filled many books but in this case we can simplistically divide it into two main schools, Delta and Chicago. The former flourished in the South (think New Orleans as a center) and flourished from the 1920's-40's. It is characterized by acoustic instruments and accompaniments that can be done by one person on guitar. As jobs migrated north in the USA, so did the workers, so a lot of folks who had been exposed to Delta Blues found themselves in Chicago. So the Chicago school grew out of the Delta school, starting in the 1940's. The two biggest differences are the inclusion of electric instruments and performance by a band instead of a solo act. In the context Casady's comment I understand a Delta Blues bass style to be very much rooted in a fingerpicker's bass so it would be notes on the first and third beats and rests on two and four. Not a lot of runs and not a lot of harmonic variation. Chicago Blues bass lines move into a walking bass line, playing on all four beats and often stray from the basic root, fifth and an occasional third.
Most rock musicians who have only casually studied the Blues think the Chicago School is all there is (or all that matters) and so a modern rock bass line will often have its roots in the Chicago style.
I can't think of any bass players associated with Delta Blues because when you go back to the beginning, the style predates the instrument.
How's that? Now let's see how many mistakes I made and what other nuances I have missed.