They probably mean that the official stance, as voiced by Zelenskyy on TV and state media, is to reconquer the whole of Ukraine, including Crimea, but everybody knows that it perhaps won't be possible to achieve this ultimate goal.

It's not private, but it is not a secret either. It just isn't the official position of the government.

I think it's good for the morale that Zelenskyy the strong leader of Ukrainians that will repel the bloody Russians whatever the odds, and Zelenskyy the prime minister with a terrible defensive war on his hands, to be two separate personas. One is more personable, but you need the other to win hearts, minds and the war.