The only people who knew about Darcy's proposal were Jane, and Aunt Gardiner, but only by her powers of observation and deduction. No one else suspected of the attachment - not even the rest of Elizabeth's family. How would such a rumor get to the Lucas family? It could have been town gossip, I suspect, in which case, it is a credit to LC, that she was observant enough during EB and Darcy's Rosings stay to suspect her nephew's feelings to take that gossip seriously.