Next door to "we don't know what we're doing yet" is "that sounds great, but I'll need to check my calendar, can I call you back tomorrow?" I hope I would remember a social engagement, even well in the future, but I might remember it as "I've agreed to go to so-and-so's wedding/bar mitzvah/graduation" and a very approximate time, like "I think that's in June."
That said, in this case, the appropriate and polite Tea Drinker would check her book and get back to you, or at most pick up the phone and say "Cousin So-and-so, I forgot to write down the date of your graduation party, please remind me." A friend of mine is dealing with something that resembles this: they have accepted an invitation to see someone be awarded a Ph.D., and the school in question hasn't yet told anyone when they will be holding Commencement. (There are failures here, but not on the part of the person receiving the degree: you can't share information you don't have.) This leaves my friend keeping a significant chunk of time open, while they wait for more information. The best they can do is tell other people "Yes, that sounds fun, but I can't promise anything," and accept that this means they may not get to attend the event even if there doesn't turn out to be a conflict.
So, while I doubt the OP's friends were in a similar situation, if only because they would likely have been more specific, such things do happen, and not only for medical reasons.