I don't have the book in front of me now but I think that was more the author's interpretation of events as only a few people knew where Amuso was and Casso was one of them.

Hindsight is always 20/20 but if Casso did drop a dime he may have hoped that throwing Amuso to the wolves would have allowed him to escape. Sort of like in one of those old WW2 movies where one soldier yells grenade and throws himself on it, thus saving his buddies. If the allegations were true Casso was the sort of fellow who would yell grenade and throw somebody else on it, thus saving himself...