The way laws work is, "Ignorance is no excuse."
However, I suspect there is no law against a concealed carry at a bank:
Concealed Carry In Bank Supported And Encouraged By Management | Gun Blog
No bank should be allowing it if federal law does not and this one does.
It may be a deterent to would-be robbers to know a bank allows it.
Many who make large business deposits of cash have concealed carry permits.
His point is that laws don't allow us to do things, they restrict us from doing things. In the absence of a law, an activity is not restricted - like carrying in a bank in Indiana.
The Texas bank that posted their sign obviously knows there is no Federal restriction on the activity but their residents can only carry concealed.
Open carry is illegal there so the sign may be the only crime deterrent they have available.