There's also the fact that the Gravemind makes it explicitly clear to the Librarian in Silentium that the Domain itself is Precursor technology, and has been the fabled "Organon," a relic that could activate all Precursor technology, all this time. Before Halo 5, we all thought it had been destroyed by the Halo Array, like the rest of Precursor technology, because of the "neural physics" they used. Apparently not, which I think is suspicious. That makes the fact that Cortana has suddenly changed all the more believable, looking at in this context.