Yeah, not gonna lie, when Tsubasa and Itsuki first change and look at each other and just say Carnage Form I was really confused how they both just...knew that's what it was called.
"I am an expert in this thing that has never happened to me before"
That particular "awakening" sequence, I thought, was pretty cheesy, rushed and nonsensical. What I think they were trying to convey was that—being controlled or coming into harmony with a greater power they have achieved within themselves—they are now able to subconsciously invoke the power they really had all along.
But everything about the actual scene doesn't convey that. Persona does the same thing with P3, P4 and P5, however it is made ENTIRELY clear that the characters, upon "awakening," have reached some heightened state of being, and it gives more credence to the fact that they would be able to say "Persona" like they knew what that was all along:
Kinda made me brace for the worst for my expectations regarding #FE's story.