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.