I don't think Mrs. Gardiner believed the engagement was secret so much as put on hold while Lydia's business was sorted out. She's been wondering what passed between Darcy and Elizabeth at the inn, and then Darcy arrives, acting as though he has the right to help the Bennet family but claiming a different motive.

She probably concludes that Darcy proposed in Lambton and promised Elizabeth that he would help her sister but since things were in such turmoil already, the couple decided not to add to the chaos. The Gardiners wouldn't expect to be told before Mr. Bennet, who has enough to worry about, and the engagement could have been announced in a few days. Elizabeth's letter claiming ignorance must have come as quite a shock.