They aren't saying what kind of gun. Zip guns have a long history of being made in prisons, although it worked for two consecutive shots that were both deadly, so it may have been a real gun somehow, zip guns usually don't work more than once, and they can often be less than deadly.
When you look at prison contraband it can come from many sources, people smuggling in things at visits, throwing things into the yard, drone drops, and prison workers smuggling things in. Civilians and LEOs have all been caught doing this to some degree.