I think Sollozzo was referring to the actual hit. But I can see your argument for having the meeting proposal on drugs. I tend to believe it was more about the actual hit - I think Sollozzo was basically saying that the hit was relatively easy and that the Don would not have been so careless with his security. As we all see in the movie, Fredo provided absolutely no protection, as evidenced with his bumbling of the gun. In the novel, Sollozzo tells Tom that Fredo could have easily been killed too, but Sollozzo didn't want any more hard feelings than necessary. I believe he said "Fredo is alive because of me" and that the gunmen had 'strict orders' not to kill Fredo.