That's a contradictory sentence, if she possesses it at a lower rank, its not the same, and thus has no reason to scale to hers.
Black Iri is Justeaze, in other words, she's the personality of the Greater Grail. This means unlike Shiki, her wishes are actually just from the Greater Grail. We don't have the Greater Grail as 1-A, which is partially due to the fact that the grail punching a hole to the root is only when it has the power of 7 heroic spirits returning to the root, at any amount below that, it can not perform this feat and lacks anything resembling that level of power, when it fulfills a wish, its not with the same level of power as with 7 souls, so its wish granting (the basis of the black grail's skill) is fundamentally not on Shiki's level, so scaling like that makes no sense
Additionally to this, as stated above she kinda does like
Lose, and unlike Shiki, she was actually trying
With those combined it is pretty hard to argue that connection to the root C scales to connection to the root A. Skills being infinitely weaker than higher versions isn't actually impossible, see Magic Resistance for instance, at the higher end we have Surtr, who is just straight up immune to basically all magic, on the lower end we have Teach, who can't even negate any spells and just takes slightly reduced damage