Looking at the charts, the Cessna brushed right up against the ground-4500 class C space. Not the smartest place to be off-frequency, in what sounded like mixed viz conditions. At the same time yeah, Austin Approach screwed up - "that traffic's through your final", when obviously it wasn't.
But this is also why you have a layered system of safety. Here, there are three layers:
1. Uncontrolled aircraft stay in uncontrolled airspace, airliners generally stay in controlled airspace (speaking generally, I know class E is controlled, but it's not positive control, and I know that the class C doesn't extend up to the class A) (successful)
2. The airliner had TCAS, as required, and the small plane had a working transponder, as required. As long as the airliner followed the RA, they would remain safe (successful)
3. The controller was supposed to provide separation on a workload permitting basis (not fully successful, the goal is more separation than was provided).
The ultimate result though was that the two planes missed each other by a fairly comfortable margin. The great thing is that the system doesn't depend on everyone being perfect to have a safe outcome, and it's been many years since an airliner has hit another plane in the U.S.
