Cindy--based on the historical record, it sounds like Joseph was indeed real. However, unlike his (half?)-siblings Anselme, Anastasie and Thérèse, there do not seem to be any official or parish records of his birth, marriage or death. The Nova Scotia archives have marriage records in Acadia for his siblings: see https://novascotia.ca/archives/acadian/results.asp?Search=castin&fieldSelect=name&Language=English
. WikiTree discourages adding unsourced individuals, but in this case I think the DCB entry is good enough.
As far as "last name at birth" is concerned, it looks like all the Baron's children (Anselme, et al.) used the surname "de St-Castin". However, their father's LNAB was d'Abbadie: Jean-Vincent d'Abbadie de St-Castin. As you know spelling was not standardized in those days and you do see variants like St-Castain, which is pronounced the same. The modern spelling is St-Castin, and I think it would be good to use this spelling for the Baron and his father on WikiTree. The Baron's common name was Vincent.
If you can read French I would recommend a biography of the Baron ("Saint-Castin: baron français, chef amérindien"), which has lots of genealogic information on this family. There is much information on WikiTree for this family that needs research and correction.