Nope, they can use just steel cables. Plus it had already happened at the same garage less than an year before: http://www.statesman.com/news/local/car-dangling-from-parkin...

The city, which inspects buildings after construction is complete, has no record of having inspected those cables since the garage at Brazos and East Sixth streets was finished in 1981, city spokeswoman Sylvia Arzola said

Using multiple strings of high-tension, half-inch cables as a parking lot barrier in lieu of concrete walls is increasingly common, according to an Austin structural engineer, and is listed as an acceptable alternative in city and international building standards.

http://www.mystatesman.com/news/local/year-old-cables-played...