I don't think it was an issue of doubt. Tsipras seemed to be looking for another bargaining chip. If the majority says 'no' to proposed changes, then Tsipras is in a better position to say 'no' and hope there will be further discussion.
The problem is that the rest of Europe has already stated that a 'no' on the referendum wouldn't lead to a stronger hand for Greece at the negotiating table. I believe that Greece will like fall out of the Eurozone
That's not really an unintended consequence for many who voted "no" in the referendum. They argue that that's exactly what Greece needs: exit from the Eurozone and print their own undervalued currency. It means some seriously tough times ahead, but it gives them a chance to grow their economy because their export and tourism industries will be far more competitive.