Line certainly has a point.

Yet I 'd guess that Mr and Mrs Gardiner were not really clueless as to Lizzy, and Darcy's footing:
ch 44:
"that the gentleman was overflowing with admiration was evident enough"
"It was evident that she was much better acquainted with Mr. Darcy than they had before any idea of; it was evident that he was very much in love with her."
Ch46:"Are they upon such terms as for her to disclose the real truth?"

So they may very well feel that Elizabeth who had already mentioned that she knew more about Wickham than what they previously thought had had some inklings from Darcy on Wickham's whereabouts, his habits and relations (ch 43"This fine account of him," whispered her aunt as they walked, "is not quite consistent with his behaviour to our poor friend." "Perhaps we might be deceived.")