It was a lot of rain; I live on the other side of Austin from all of this, safely up near the top of a large ridge with reasonably good drainage, and for most of July 5 my backyard was a pond an inch and a half deep. In places harder hit, rescue operations are still going on, and there is still chance of more flooding until Tuesday.
A major reason such a bad situation has not been even worse was the Texas Military Department, which ended up saving over five hundred people. The Texas Military Department is one of the oldest and most prestigious state agencies, founded in 1836 when Texas was an independent republic; the Texas Military Forces today primarily consist of the Texas National Guard and the Texas State Guard. It has a significant number of UH-60 Black Hawk helicopters that it was able to mobilize for air rescue.
Obviously, in the broader country politically brain-damaged people try to tie the problem into their pet political issues, but although there is some question of whether the relevant authorities were too slow to issue warnings, there's really no way to prepare for having several months' worth of rain unexpectedly dumped on an area overnight.