Did they ever find the guy who was allegedly carrying a weapon? If not, I would say the odds of a liberal plant are very high - who do you know that would be carrying a weapon openly just for the heck of it in an area where guns are banned?
We have no idea if there ever was a firearm. The only reports that I have read have the children saying that they thought he was carring a gun underneath some cloth. Very vague and unsubtantiated.
Guns are "banned" there only by University policy, which has no force against someone who the Univeristy cannot sanction. If the person was not a student or faculty, they would have no power to sanction them.