Helen, he has uttered that there's a comparison in case of "that" and there's no comparison in case of "so".

- Chew faster!
*the other guy starts chewing at the speed of a machine gun*
- No, not THAT fast. (not fast like that)

Medved and Paul G. fought because of Helen. She was that beautiful.
(She deserved to be fought over.)
If he said "she was so beautifil", it would have kinda disconnected the sentences, like:
"they fought over her, oh, btw, she was beautiful".

See the difference?