I agree with your theory. He had to have used the cm to break out of it. I also believe that even though Itachi's Tsukiyomi can last a second in the real world, he prolonged it during his fight with Sasuke in order to give him enough time to break it. After all, he wasn't serious and just wanted to push Sasuke to his limits so that Oro would come out. That was brought up in this thread by TBogard:
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I don't see any reason why Itachi wouldn't be able to do so, also Zetsu noticed that it was occuring, so it must have lasted more than a second. Obito also mentioned in a manga panel that every part of the fight was planned, even the contingencies & in the next panel, there is a flashback of Itachi's Tsukiyomi being broken. I also don't see how one could break Itachi's Tsukiyomi if they looked directly into his eyes and he was to make it last only a second in the real world and as long as he wanted to in the genjutsu. How can you even have enough time to gather enough chakra in order to break a genjutsu that lasts only a second?