I don't know where I read this and it didn't say that Prue's name is "Prudence Melinda Halliwell" , it was about that Piper and Leo wanted to put their baby's name that but when she gave birth to the baby , tehy didn't have a girl and the baby was boy so they couldn't give that name to a boy
Anyway ,, I think Those are all fanfictions and some people read those fanfictions and believe them , just like the way some people believe that Piper , Prue , Paige , Phoebe are for real