Notice how Chirithy's statement is a question though?
Anguis' answer to this question would probably yield the more interesting bits, not to mention that Anguis "suspecting" something is still no proof at all.
It's not only the Foretellers themselves that cannot really continue to trust each other. We, as the audience, also cannot really take any of the Foretellers and state "this one's statements are trustworthy" because we do not have any indicators towards which behaviour would actually point at the "traitor".
So we cannot really determine yet if the statements of any of the Foretellers are trustworthy at all.
Chirithy itself is also suspicious as is the sixth Apprentice, the Grand Master and even Ephemera isn't out of the loop yet.
It also heavily depends on from which viewpoint the "traitor" is actually observed and what or who exactly this traitor even betrayed in the first place.
Vulpeus recruiting and collecting children from across all unions can under circumstances also be counted as treason to the designated roles of the Foretellers, as she clearly seems to act out of their usual mandate.
I'll also advise to not attach the Foretellers to the Rebirth-theory too much in this regard as this can lead thought-chains astray.
It is true that Riku was the only one to ever willingly betray the light and his friends (in KH 1), as Terra never intentionally caused his friends any harm while Kairi, Aqua and Ven are practically above any suspicion.
The Foretellers however are NOT these characters so associating them too much with them can warp the perspective.