Could it be that they were talking about the actual Corleone family -- as in Michael, Vincent and their immediate crew? Zasa had taken over the Corleone operation, which I am guessing included drugs. And maybe Mike made it difficult for Zasa and crew to keep dealing in them, which is what the line is referring to.

Either way, it is sloppy writing, but I think a lot of sloppy writing in the series has been dismissed ("Michael Corloene says hello") because we've been able to tie it into different scenarios. We could probably do the same thing here.