I've always interpreted this as the Gardiners having no reason to believe that Elizabeth and Darcy had come to an "understanding". A secret engagement was regarded rather seriously in those days.

And, on the other side, since Darcy has ample fortune of his own, and they've convinced themselves that he has a good character (chpt 44), so there's no reason to interfere to prevent an engagement like Mrs Gardiner had in Wickham's case back in chapter 25.