I believe Fred did contradict Capt Harville's assumption (that he and Louisa were engaged) aloud. It was also the proper thing to do, since he and Louisa were really not engaged. Even if a gentleman was intending to ask a lady to marry him, but had not yet proposed, and his friend assumed they were already engaged and said so, he had to tell his friend they were not, (or at least not yet, but that he hoped she would accept him). To let an engagement be assumed where there is none would be unfair on the lady, since she might not accept him. It could also unfairly keep other suitors away from the lady. I don't believe Capt benwick wold have gone ahead with courting Louisa if he thought that she and Fred were engaged, he would have been too honourable to his friend to do so.