That doesn't necessitate him being important, it could just make him a buffoon, like Joey.
You mean the guy who achieved to infiltrate a whole fortified place, disarm all traps, while covering Gracie, all this without being noticed a single second ? I wouldn't call him a Buffoon, even jokingly.
He's a comic factor, there's no doubt about this, but Cordia was probably half serious when she wondered if he is an unknown child of Wilfred. He's the least best of the Elite, average in everything, yet he still achieved to reach the Elite, and he's so forgettable that his stealth mastering past totally unnoticed ; and would still be if Gracie wasn't with him.
For all she knows, he could be the same rank as them.
I doubt.
Even forgetting the dress code, it's not like there's a new Capo every week.
It need years for someone to reach this rank, and you're noticed by the other families way before it happen. This apply even more if you achieve to reach this rank faster than anyone normally do. Therefore, when you're part of the intel for a family of assassins, and the second daughter of the family, you know this kind of things ; at least for the five families. Perhaps that she don't know if MC is a soldato or an Elite, but she know for sure that he is ranked below Caterina and Amata.
If you want, look again at the fight inside the building. The Capos were able to tell that it's Amata and not Amita. Do you think they are smarter and more informed that Siobhan ?
But I don't remember Siobhan knowing that he lied about his fight? Ono saw it. Did Siobhan?
She don't need to have seen it, as well as Caterina and Amata don't need to hear Ono describe what she saw.
The fight happened inside a building, and was against a Capo who was protecting a facility from his family ; in top of that it's a family of assassins. You need a really big amount of luck to just ran away from him, and even more for him to then totally lost your track. Siobhan should have a really lack of faith in the Capos of her family, while Caterina and Amata should be really naive, to believe that it's possible. What effectively happened is unknown of Siobhan, and would have been unknown of Caterina and Amata if Ono haven't said it, but it's clearly not by running away that MC survived this fight.
Plus, neither Caterina, Amata, nor MC would have been so at ease if they knew that he was still there, searching for MC ; Someone trained to gather information, like Siobhan is, couldn't miss this detail.
Unless you wore the soldato outfit, which obviously no-one wore 'cos lower stats, how would she possibly know the rank of the unknown person in front of her?
As Hopes said a long time ago, it's not just for a question of renders that MC don't wear the selected clothes when officially on mission.
Even if you're just a soldato, you're proud of your family, and you wear its colors on mission. It's only when you reach a rank where everyone know who you are, and therefore what is your family, that you can dress like you want.