The district that Kelly won includes the part of Austin that slops over into Williamson county, which is still much more conservative than Travis county. That explains her win as much as anything.
Also, the other City Council runoff race held yesterday was won by the lefty incumbent, so it doesn't really look like there has been any seismic shift in Austin politics. At best it will be 10-1 for the lefties, instead of 11-0.