This is a really interesting thought. I read that bit again after reading this, and I think you may be on to something. It is not even really his rank, or lack of it, that Lady R objects to so much. It is that she does not think he has the means, or the character, to make Anne happy. In some ways she is making the mistake of projecting on to Anne, because she loves her, her own preferences. But there is also the fact that she saw her friend's marriage and its failings, and must have known her friend not to be happy. She must have seen Anne's engagement as a similar mistake by a young woman in her choice of man. Very different situation - but we may have to wait to make a judgement about how different the characters of the men are; we know, for example, that Wentworth had 'spent freely' all that he had made in the war so far!