Why? Because he uttered the impossible. That if a majority of Quebecers voted to seperate from Canada, the entire province may not be allowed to leave Canada as a whole. There may in fact be regions of Quebec where the majority do not want to seperate. And those sections may be carved off of a greater Quebec.
Of course this created a furor and the Charest camp was forced to issue a retraction. Of course Jean didn't mean that Quebec could be seperated after seperation. What he meant is that Quebec is indivisible.
It would seem to me that one of the strongest arguments against Quebec seperation, would be that if Quebec was allowed to vote for its seperation from greater Canada, then regions within Quebec would also have the right to seperate from greater Quebec.
Now I don't have an opinion here, or a declaration to make, but out of curiosity - this partition would seem to be a very strong argument against seperation, why is mentioning it forbidden?