Ran knows too much

Well, as Yukari's "weapon", Ran would've been in the (extreme) front row of Kaguya's defeat during the events of Imperishable Night (assuming a Reimu playthrough), so she should know all about Kaguya/Mokou.

In fact, it's a little strange that Nitori wouldn't know, being as information on who Kaguya was should have disseminated through some of the canonic cast herd get-togethers, like Silent Sinner in Blue, even if only through the Tengu like Momiji.

NWSiaCB said:
In fact, it's a little strange that Nitori wouldn't know

Canonically the fact that Kaguya is a lunarian (and immortal) is not widely known (if Aya and/or Akyu know they at least write misinformation.)

But Ran knows about Kaguya (and Mokou).

This is much later than the previous comments, but I don't think this is completely following canon. In the beginning, Nitori seemed to speak to Reimu and Marisa like she had never met them, and they didn't appear to know her either.