I first saw that lineup in Keith Badman's book a few years ago. One can surmise that WGGT was around for a while prior to vocals being laid down, hence the backing track's inclusion on the record. Lastly, some think that song is wonderful, including myself.
I LOVE When Girls Get Together.
It sounds like an Everlys song.
I'm not sure it would have fit in on Sunflower, but I think it should have been on an earlier album for sure.