if Emma were to tell Harriet (although she may think it) that Mr Elton is a social climber in wanting to marry her but that would not have been the case had he aspired to Harriet (therefore implying Emma's social superiority over Harriet). I know many of Emma's actions imply that idea anyways, but it would have been awkward to say it straight out like that, so that particular flaw in Mr Elton could be kindly skipped.