Maybe, but that bugs me. Intellectus is supposed to be more absolute than that, I think. Even if Harry's limited version isn't, I mean... how far does the effect extend? If a non-human being with full Intellectus tells an adult human about them, does the human just instantly forget about it?
Probably would forget, unless they had some other way to /make/ the memory stick. And Id assume harry could still spot one with his Sight if he actually knew what he was looking for, though a lot of the time he actively ignores most of what his Sight shows him in basic self-defense. Alfred's Intellects has always been the most limited example of it from day one, and is particularly focused on his Inmates, which I think is a lot of why he was never very practiced at recognizing mortals. But if one of the Creepers were to get locked up in the Well I think he'd start taking notice a lot more. Though its a good point that Creepers have very little reason to venture out to an island with no children to target; they strike me as more a zoo and candy store crowd.
Angels have full intellectus. What happens if Nicodemus asks Anduriel, "what supernatural entities are within one mile of me"?
Personally Id dial down the radius just to avoid getting overloaded
But I see your point, anyone else Id say he may simply not be /allowed/ to reveal that aspect of Reality, it seems specifically reserved for Children. But he's already breaking the rules, so may he has told him, maybe he's aware that they exist even ifhe cannot sense them directly or retain personal memories of them. Or maybe a Fallen bonded as long and tightly as those two can just provide/overlay the sensory capability like how Lash let harry ultrasound see in the Dark.
EDIT: Even worse... what if a kid writes about the haunts and Ivy picks up the information? When Ivy hits whatever age makes you forget about the haunts, does the information get deleted from the Archive, or reclassified from "nonfiction" to "fiction", or what?
ooh, that's dark... She's likely a special enough case that can just KNOW that sort of thing, retain it without risk of revealing it and damaging whatever Cosmic Machinery is in motion. Like a Fallen, if anything could keep you from Forgetting it would be (cue ominous voice) THE ARCHIVE...