True. If she were honest about all this she could have told the story Casey told her, but with less elaboration. Something like, "She told me they went to a convention during these days. (Insert names of people and dates) They had to stay an extra night. One of the cars was in an accident, the people were injured, (give date and name of hospital) so they stayed a few extra days." Quite simple really for a fictional tale. It would have established that Casey and Caylee were not at home during that week. She was far too elaborate with the fictional tale which, in effect, made Cindy look guilty as hell.
My theory is Cindy went into that meeting planning to stall with these outrageous lies of Casey's, thereby, killing time and not getting down to the true facts. Remember, she was adamant she had to leave by a certain time.
When Yuri jumped in, (which is very normal in cases like this) he was injecting an element of surprise and wanted to get Cindy's true reactions to Casey's lies ("none of these people exist"). Cindy went into combative, defensive mode, when that didn't work, she played her "poor grieving grandmother" trick. She's not stupid, just conniving like her daughter.