I can’t censure Mr. Knightley for believing Harriet is a simpleton meaning “an ignorant, foolish, or silly person” (dictionary.com) because it seems a pretty accurate description of Harriet. Even Emma admits Harriet “is not a clever girl” (8). When Emma says Harriet “ does not deserve to have her understanding spoken of so slightingly” (8) I imagine she has forgotten their conversation about Mr. Elton’s riddle in Ch. 9 or Harriet’s inability to decide where to send her ribbon and plain muslin in Ch. 27.

Harriet is ignorant and she is inferior to Emma in taste, sense and understanding. Emma is flattered by Harriet’s inferiority as in “to feel satisfaction with (oneself), especially with reference to an accomplishmen (dictionary.com). Harriet stimulates Emma’s intellect to concoct a gentleman father and arts to bring Harriet and Mr. Elton together. I don’t think those pursuits occurred to Mr. Knightley in Ch. 5. I don’t see that Harriet inspires any other intellectual stimulation; certainly Emma does not read more herself as Mrs. Weston forecasts in Ch. 5 instead they collect riddles and Emma day-dreams:

Her views of improving her little friend's mind, by a great deal of useful reading and conversation, had never yet led to more than a few first chapters, and the intention of going on to-morrow. It was much easier to chat than to study; much pleasanter to let her imagination range and work at Harriet's fortune, than to be labouring to enlarge her comprehension or exercise it on sober facts; and the only literary pursuit which engaged Harriet at present, the only mental provision she was making for the evening of life, was the collecting and transcribing all the riddles of every sort that she could meet with, into a thin quarto of hot-pressed paper, made up by her friend, and ornamented with cyphers and trophies. (9)

On what facts should Mr. Knightley concede Harriet has a chance to a connection higher than Robert Martin? I think the possibility Harriet is of great wealth or noble parentage to swipe away the stain of illegitimacy was always a hopeless business. If such grandeur had been hers IMO she would have been housed in an institution where she could be trained and educated for a better life than the second set of Highbury. Also I do not remember that there are any other young ladies who are gentleman’s daughters at Mrs. Goddard’s school.

Harriet “proved to be the daughter of a tradesman, rich enough to afford her the comfortable maintenance” at Mrs. Goddard’s boarding house so Mr. Knightley’s conclusion Harriet was maintained in the way of life she was born to was correct. It may charm our modern sensibilities to agree with Emma that Harriet “is not to pay for the offence of others” (8) but it is completely unrealistic and Mr. Knightley is too pragmatic to ignore facts for the sake of Emma’s vanity.