First off stop dawging on Kurenai for using genjutsu on Itachi. Itachi wasn't widely known for his genjutsu abilities he was widely known for making his clan past tense. Secondly, Might Duy used THE 8TH DAMN GATE on the 7 swordsman of the mist.
What doesn't make sense to you exactly? The Uchiha are well known for their genjutsu, several characters make mention of this. Specifically, Itachi is internationally praised as being comparable to Shisui's visual prowess.