The thing about it that I have focused on is, given the time in between the warning and evacuation, I wonder if they conducted a preliminary search that uncovered something substantial enough to call for an evacuation.
Logistically, the campus in Austin is far more accessible than the actual capitol. I can see why it would be targeted strategically, furthermore, the new age in terrorism prides itself on publicity. It's not too far out of the realm of possibility to think they would "call to warn". Look at the media attention potential it creates. Bomb or no bomb, if its a legit terrorist group, in this case Al Q, they'd chalk this up as a victory of sorts in their own twisted and deranged perception of significance.
Lastly, eff terrorists.