It has nothing to do with definition but with the simple fact that there were two completely different arrangements/versions of Southbound. Your link (from the Old Waldorf, March 31, 1979) is an example of the original version (as it was demo'ed and played on stage) while the studio version is like a completely different song, with a different riff, a different groove, and much shorter.
The studio version was never performed live, neither before nor after the album.
The original version has always been very much the same with only little variation (sometimes a bit faster or slower, a drums intro added in late 1979 , one riff missing in the break in the middle on the demo, that's all I think..)