I, too, thought Sonny was being skeptical until I saw otherwise in the book. I think Caan misread the line a bit.

I think that Sol's calculus changes if Sonny keeps quiet, but not dramatically. Vito's the one who thinks differently among this group. It can be assumed as unlikely that Sonny would be as hard line against the drug trade. Sol's main concern with respect to Sonny is how to counter his attempts at revenge, and those would come regardless of Sonny's thoughts on drugs.