It had been 10 to 12 years since Mrs Gardiner had really lived in Derbyshire and Darcy's father had only been dead for 5 years.

Wickham seems to be very careful in finding out how much the other person knows about Darcy's family before he tells his "tale". He is very careful then to say just enough to make the story seem true. Wickham and Mrs. Gardiner were in agreement on Pemberley and the late Mr. Darcy--so he uses this as a way to gain her sympathy for his misfortunate treatment by Darcy.

I think Mrs. Gardiner may have convinced herself that she remembered an incidence that backed up his story--but I don't know if she would have said as much outloud. I think she may have "acknowledged" it more with a smile than words?