Completely irrelevant... There's a Manga link of Kirabi, a shinobi from Kumogakure, commenting about Shisui and calling him the most powerful Genjutsu user of the Uchiha Clan.
Even if we only know of 1 Genjutsu from Shisui, you don't become hailed as the most powerful Genjutsu user of the Uchiha Clan if you can only use the technique once a decade... He wouldn't even be able to prove he can actually use the technique until 10 years later, which means once he uses the technique - He has a time frame of 10 years where he has established himself as the more powerful Genjutsu user of the Uchiha Clan, with fame that spreads across the world to the point where Ao knows of him because they fought in the past and Kirabi knows of him.. Even though Shisui has been dead for 10 years.
And also, Itachi didn't even disagree with the comment or say anything which could even hint doubt.. He continued on speaking and praising his MS' ability.