I don’t think it is dismissive for Mr. Knightley to say Harriet has no advantages of birth, connection, understanding, fortune, culture or accomplishments to attract men of a higher social status than Robert Martin. It seems a completely rational evaluation of Harriet’s situation and in fact Harriet does not attract offers from gentleman as Emma predicts:

Nothing so easy as for a young lady to raise her expectations too high. Miss Harriet Smith may not find offers of marriage flow in so fast, though she is a very pretty girl. Men of sense, whatever you may chuse to say, do not want silly wives. Men of family would not be very fond of connecting themselves with a girl of such obscurity -- and most prudent men would be afraid of the inconvenience and disgrace they might be involved in, when the mystery of her parentage came to be revealed. (8)

It seems to me what you are saying is that unless Mr. Knightly buys into Emma’s fantasies of Harriet’s beauty and good humor nabbing a gentleman then he must feel she should not have the opportunity to marry up. If that were so why does he approve Mr. Martin’s asking for her hand? He believes Mr. Martin is in every way Harriet’s superior and by approving the match he is approving Harriet moving up in the world:

My only scruple in advising the match was on his account, as being beneath his deserts, and a bad connexion for him. I felt, that as to fortune, in all probability he might do much better; and that as to a rational companion or useful helpmate, he could not do worse. But I could not reason so to a man in love, and was willing to trust to there being no harm in her, to her having that sort of disposition, which, in good hands, like his, might be easily led aright and turn out very well. The advantage of the match I felt to be all on her side; and had not the smallest doubt (nor have I now) that there would be a general cry-out upon her extreme good luck. Even your satisfaction I made sure of. It crossed my mind immediately that you would not regret your friend's leaving Highbury, for the sake of her being settled so well. I remember saying to myself, 'Even Emma, with all her partiality for Harriet, will think this a good match.” (8)