Hmm... what did he lie about?! I do wonder! Something he "told" us in DA2?
Well, he did start DA2 lying his arse off to her about Hawke tearing Ogres limb from limb, his Tony Montana moment with Bartrand, as well as his casual omission of the events of MotA and Legacy?
But if he did lie about something in the game that didn't happen as we saw it in-game, it's probably him stretching the truth here and there so that it'll come off as less incriminating for everybody.
This might be a little controversial, but I wouldn't mind to learn that Hawke actually was taking steps to attempt to oust Meredith from power and was far more involved in the various Act 3 conspiracies than we were lead to believe. Varric was just lying about it because it would have made Hawke seem far more culpable if they were also involved in a conspiracy to incite rebellion and sedition against Meredith, even if had absolutely nothing to do with Anders' mage rebellion putsch.
It would at least finally satisfy those who weren't happy with the lack of player agency in the game and appease those hated Hawke as a character because they weren't proactive enough. Call it a retcon, asspull or author saving throw, but I think it'd make Hawke a far more interesting character, if it turns out whether you played them good, bad and snarky, Hawke was actually far more complicated and devious than we ever imagined?