I am not sure that "listels" is Italian in origin. It is a word associated with tiles and moldings and other architectural details (like M in FW said)
In the dictionary (Webster's) an alternative def. is "filet"
So I assume it is sliced fresh mozarella. Because if you have ever eaten a Pizza Margherita, that is what it is.
Wikepedia is a notoriously sketchy source of information, as it is user-compiled. It often just restates what people glean from other sites. The "mozarella in listels" reference can be found in other sites, too. Probably just copying one another without giving it an ounce of thought.