The DoJ at the time explicitly denied Assange being under sealed indictment, which, if he was, it should not have.

(In general, the US government is not supposed to explicitly lie to the American people, hence the "I can neither confirm nor deny" line that comes up when an issue is classified; you can't say P, because you can't reveal classified information, but you also can't say not-P because you aren't allowed to lie in an official capacity.)