Indeed. Mr K is not an egalitarian. he is probalby in some ways more sympathetic ot hte idea of people "rising" in the world than Emma is generally, but he problaby sees it as OK if they are rising by their own efforts, and if they do it gradually, rather than someone rising by marrying into a class they do not belong in and are not going to fit into. If Harriet was a very clever girl, and Emma took her up, improved her social graces (and if H had had a good education and was "street smart") and she had found a suitor at say the very lowest end of the gentry, who had a little money but no grand social position to keepe up, I thtink that Knightley might not condemn it.. but he thinks clearly that Harriet isn't very bright and is likely to have her head turned by Emma giving her notions that she is a gentleman's daughter. and that given that nothing is known of her fahter or what moeny she might coem into, I think Knightley fears that she's not going to catch a genteel husbadn without money and a known heritage, so she's beign set up for a disappointment.