"That's really brave of you, Antoine."
Translation:C'est vraiment courageux de ta part, Antoine.
si courageux / tellement courageux = so brave très courageux = very brave vraiment courageux = really brave
"Si" and "tellement" appear to be synonyms, but the rest are alternatives. You're right that they're close alternatives, but the point here I think is not to use close approximates but rather the actual word itself.
I guess the confusion is due in part because in English we don't properly distinguish between very, really, and so. Can't assume the French do as well.