In the case of the "No drugs" myth, I think it is another convoluted story. Similar to the discrepancy over the year of when the books were closed. My example would be Sammy The Bull in MSOA saying the books were closed in 1955 ( if you remember, Anastasia and Scalise were selling membership. So maybe the info Sammy got about their particular Family was correct) but most saying 57-58 after Appalachian (which would make more sense).
Similarly, Michael Franzese said they were told not to deal in drugs (to my knowledge, the Colombos weren't involved on a scale of the other Families.)

So I think it's the fact that, informers make statements about their respective borgatas and it is taken as a blanket statement for all the Families.
I also believe, that each Family had their on separate drug "thing" and it wasn't a collective effort. That's where the confusion comes in.

And maybe, what they were really saying is, You Can't deal drugs like Vito Genovese or Joe Bonanno ( on a wide scale) but if its quiet and in the background. Money is Money.