You are right. They made their fortune in fast food franchising. Incidentally, they have come out with their own public statement.

https://nypost.com/2023/08/15/michael-oher-accused-of-15m-shakedown-of-tuohy-family/

As it was writer Michael Lewis, not the Tuohys, who wrote "The Blind Side book on which the film is based, it stands to reason that their take from the film was modest.