So you asked an employee a legal question and got a bad answer - it's not a preemption violation for a random employee to make a mistake of law or lie to you.
He was the front security. By his answer, he would take action against someone entering the building open carrying, in violation of the person's rights and state pre-emption. This has not actually happened
to me, of course, which is why I said I am going to follow up with the building administration.
As to the question about why I asked, the answer is that I would rather have a discussion with the building administration and convince them to change their policy, than by walking into the building carrying and being arrested. At that point I have no choice but a lawsuit
to change their minds. I would rather avoid one.
I might still attempt that approach, open carrying into the building. I thought it would be a cute trick to concealed-carry into the building, go about my business, then right before I leave, convert to open-carry. Walk out past the guards, see what happens!
My thinking is that they'd have a greater chance to win against me by preventing me from entering via trespass laws (even though since this is a public building they can't), than by attempting to arrest me while I'm already inside, obviously on legitimate business, and causing trouble for no one, and on my way out.
But that's why I'm here asking for advice. I want to take the approach most likely to achieve a positive result.