[We (and Elinor) know that the colonel did not embellish the story at all.]

Not only does he not embellish, he holds back on details about which we'd be avid to hear! That passing mention of the duel, for instance---so many first time readers don't even realize there has been a duel because of Brandon's discreet manner of speaking. (Ah, those few sentences that launched a thousand fanfics . . . *g*) But the more often I read Brandon's confession, the more impressed I am with his self-control; even though the whole story is unspeakably painful to him ("even now, the recollection of what I suffered") he does not dwell on it to exact pity from Elinor---even though he certainly does receive it from her. Willoughby, however, can hardly stop dwelling on his own suffering long enough to consider just how much harm he has done. Plenty of "embellishment" there . . .