Cobb was an educated guy from a wealthy family, when most players at that time were not. The "rubes" resented him and he hated them right back. He also hated the other team, umpires, ownership,probably the fans that gave him a hard time as well. His own father thought a career in baseball was beneath him. He was 18 when his mother shot him which drove him to a maniacal/hostile will to succeed and prove that his father was wrong.
So basically anyone writing about him back then would probably have found numerous people in and around the game that hated him and would be happy to share/invent stories about him. I think Stumps book is a little accurate, it just leaves out a lot of the good things Cobb did and the context in which some to the negative stories happened. Which only look worse as time passes and the social norms of the day are forgotten.