According to the KPRC article (link is upthread), he was released in less than 30 minutes, told the attendant press that is was just a misunderstanding, and made several pro-Trump statements. Make of that what you will, but he was in fact released pretty quickly. No word of what happened to the pistol.
This is Texas. He could have been carrying legally. If there were no 30.06 or 30.07(*) signs at an entrance to the parking garage, then there's nothing to prosecute him.
I'm not saying it was a good idea... just that it wouldn't have been illegal.
(*) For those not in TX, these numbers refer to sub-sections of the Texas Penal Code on Criminal Trespass. The law mandates a sign of a specific size and specific words to be posted at the entrance of private property, if the owner wants to prohibit concealed or open carry of a handgun, respectively.