Ah, grasshopper. You ignored the first part of the statute:
"810.08 Trespass in structure or conveyance.--
(1) Whoever, without being authorized, licensed, or invited, willfully enters or remains in any structure or conveyance,
or, having been authorized, licensed, or invited, is warned by the owner or lessee of the premises, or by a person authorized by the owner or lessee, to depart and refuses to do so, commits the offense of trespass in a structure or conveyance. "
By prohibiting entry to anyone in possession of a firearm, the owner of the establishment is rescinding any implied authorization, license or invitation to enter. If the sign promises arrest and prosecution for entering while in possession of a firearm, violating it could result in not only your arrest, but your conviction for trespass while armed. Bye bye CWL and possibly your entire firearms collection.
That is, of course, if the owner doesn't think you are a robber and shoots you dead. But, do what you gotta do.