I'm a bit confused about something Kate says

I'm new to the show and rewatching season 2. In her conversation with Lady Danbury in episode 1, when describing her family's dire financial situation that they've lived under since her father's death, Kate says that if she could marry for the sake of her family, she would. However, when Anthony first proposes, she rejects him.

His fortune would secure her family's future. If she had accepted, then Edwina wouldn't have to marry for money which, she still would have to, with the full knowledge that she has to this time.

I understand emotionally why his proposal was bad and why she needed to say no in the context of the later episodes. However, I don't understand why they left that line in the first episode, and it's going to bother me. Is this something from the book?

Thank you for your help!