"Really really" has only a few syllables and those syllables sound repetitive. When isolating the vowels, it's like tapping the same piano key twice "EA -> EA".

"Categorically" has a lot of syllables with some alternating vowels mixed in, this makes it easy to say and easy to understand when spoken. Going back to the piano metaphor, there is a bit of a tune to it now "A -> E -> O -> I -> A".

"Categorically" exists outside the vocabulary of children so it sounds more intelligent.

"Really really" elicits the memory of something your friend in elementary school might have said when insisting they "really really" want your Charizard card, not very convincing, maybe even annoying.

Finally, "categorically" has a bit more weight to it than "really really", perhaps making it closer to a "really really really really" or a "super duper" to those in the know.

"Categorically" encompasses an entire category after all, those are usually super duper big.

In philosophy, at least Kantian philosophy, "categorical" means unqualified and universal. As opposed to "hypothetical", which means given some presumptions.

Not sure if TFA uses it in this way.

See: https://faculty.philosophy.umd.edu/SKerstein/categoricalandh...