Thanks for posting, but not sure if I fully understand it.
Is the jar some sort of metaphor? For what exactly? It seems to be all about war, both real and imagined (or real in some other dimension) by Rhett's mother. But the different elements don't come together for me...
Also, sometimes there are too many stories that are told from a retirement home.
Perhaps there is no metaphor at all.
Perhaps this time, a portal to another world just took the shape of a cookie jar.
As for why is it filled with cookies, it is because it IS a cookie jar, so it is only natural that it should have cookies.
If the portal had been a shampoo bottle, it would be filled with shampoo.
All of that is just my way of saying there is nothing to "understand", because the story does not show the origin of the jar or the reason why it is shaped that way. It is just something that is, leaving us only the rest of the story to enjoy.
Now, once we simply accept the portal is a cookie jar, we can focus on the feelings this particular cookie jar brings, both good and bad.