I thought Miss Bingley believed Mr Darcy was attracted to Lizzy who was attracted to Wickham. I'm inclined it was a 'farewell' letter directed to Jane to emphasize Charles Bingley had no special regard for her and Bingley was moving on now. It was always clear Caroline had no wish Jane become her sister-in-law. Actually, I wouldn't be surprised if Mr Darcy was contemplating some new furniture, but don't want to jump ahead in the B.R... Caroline probably remembered this detail when writing, and slipped it in to make it appear some marriage may transpire just to be mischevious.