and he says that almost in secret sibling language to Elinor. I think Henry was banking on the fact that Catherine would not follow his description as a reference to her specifically because Catherine is everything he describes her as, and she attributes that to everyone else, too. Frankly, it seems to me that the comment passed right over Catherine's head, but that's not necessarily to her detriment.

H & E both know that their father (and brother) are concerned mostly about money. I think they are in a wonder at Catherine who CAN think the best about everyone. This is funny, too:
“Are they a wealthy family?”

“No, not very. I do not believe Isabella has any fortune at all: but that will not signify in your family. Your father is so very liberal! He told me the other day that he only valued money as it allowed him to promote the happiness of his children.” The brother and sister looked at each other.

(I could see them reading each other's thoughts - can this girl's belief in their father's good wishes be for real? To believe our father's comment about money? Yeah, right!)

And then Elinor says (possibly to at least try to carry Catherine's logic through):
“But,” said Eleanor, after a short pause, “would it be to promote his happiness, to enable him to marry such a girl? She must be an unprincipled one, or she could not have used your brother so. And how strange an infatuation on Frederick’s side! A girl who, before his eyes, is violating an engagement voluntarily entered into with another man! Is not it inconceivable, Henry? Frederick too, who always wore his heart so proudly! Who found no woman good enough to be loved!”

Elinor is taking Catherine and the news she received seriously - trying to consider whether Catherine's notion that Captain Tilney must have proposed to Isabella. It's Henry who knows his father and brother too well to think that is possible, but is really subtlely teasing his sister about her taking Catherine's news seriously, Elinor who also knows their father and brother well. Henry grabs the goodness logic of Elinor's last sentence about their brother in his next comment (that Catherine would follow the logic on from her POV and Elinor's comment):

And Henry's reply:
“That is the most unpromising circumstance the strongest presumption against him. [Capt. T found no woman good enough to be loved - but if he proposed to Isabella, he must have found Isabella extremely good!] When I think of his past declarations, I give him up. Moreover, I have too good an opinion of Miss Thorpe’s prudence to suppose that she would part with one gentleman before the other was secured. [Henry voicing something Catherine might think] It is all over with Frederick indeed! He is a deceased man — defunct in understanding.
Followed by the quote I initially gave.