You can accuse a leak to be partial, non-representative, timed to harm a party, etc.: that’s what people criticised when Russian secret services leaked emails from the presidential campaign of Hillary Clinton. “True” isn’t enough of a criterion for political coverage: “representative”, “clear”, “informing the voting public” at better objectives.

In this case, it’s an internal document that is proving that the government is not candid about a threat that they are waving.

If (like people who support Boris Johnson) you think that a credible threat of leaving without a Deal helps negotiate against the European Union, that leak can seem political.

If (like people who oppose Boris Johnson) you think that a leaving without a Deal is irresponsible, having official confirmation of that fact is validating.

Yes it does, because these are predictions about future events and thus cannot (yet) be true or not true.

The civil service has previously published reports like this. They turned out to be totally wrong. There's a serious and important element of cry wolf here, especially because the previous cabinet that the report was written for were working against leaving. Lots of stuff coming out in recent weeks about how ministers were saying they were preparing and ready to leave without a deal but actually blocking attempts to prepare and telling foreign leaders they'd never leave.