That it's "Based On..." has nothing to do with the way that it tells the story. It is a strictly "Paint By Numbers" version of the characters whose story it tells.
"Based On" doesn't mean you are even remotely close and not telling a story without much of a connection to reality.
No one has said that.
That said, the idea that those two films are not painting a pretty specific version of the ethnic group that founded the Mafia is on shaky ground.