Kreia did know her - she said so to the Exile herself. Kreia has a, ahem, talent for "misinformation", though, and I could easily believe that she would mere say that she knew Kae if she is indeed Kae herself. Note also how she refers to the third Sith Lord, who cannot be revealed too soon. That third Sith lord is clearly Kreia herself, so she obviously talks about herself in the third person. She could very easily do the same on the subject of Arren Kae, methinks.
However, there are a few clues to suggest that she is indeed Arren Kae.
1. None of the masters ever mention Kreia by name.