He believes Louisa to be intellectually inferior to Captain Benwick, not a choice he would make for himself - like his sister and her husband he wants a marriage where they can meet as equals. Having said that...on this reading I've been questioning how smart the Admiral is

Austen does enjoy having characters contradict themselves - see Mary's comment in the earlier part of the letter about them not improving as neighbours

All the changes indicate that the Crofts are practical, thinking of the servants convenience - they can get their own umbrellas rather than sending for them, the door was a nuisance - and not as obsessed with appearance as Sir Walter. They even move the looking glasses themselves.

Captain Wentworth barely knows them either. With the Admirals views of "anyone will do" it's surprising that he has such a happy and well matched marriage.