I'm sorry jno, but I just don't read it that way. Pretty ruthless paraphrasing of the Wikipedia text gives -
Sweeney Todd ..appear(s) in various works starting in the mid-19th century..... In some versions of the story.. his lover, friend, and/or partner in crime.. is variously Marjorie, Sarah, Nellie, Shirley or Claudetta. ....In most versions of the story, Sweeney either helps or hinders (sometimes both) the love affair of a young woman. Sweeney Todd may not be a wholly fictional character. In two books, the.. writer Peter Haining argues that Sweeney Todd was a historical figure who committed his crimes around 1800. Nevertheless, other researchers who have tried to verify his citations find nothing in these sources to back Haining's claims. There is, however, a similar story reputed to have occurred on the Rue de la Harpe in Paris that likely influenced the stories of Todd.
I don't find that an unequivocal vote for a non-fictional character, rather the reverse.