I'm one who when hosting a pot luck always goes through the line last. So I can't imagine the food running out without my realizing it. But honestly, I'm also in the group that says I've never seen a pot luck run out of food. Always the opposite. Now, one or two dishes, if extremely popular or if not enough was brought, might run out. But completely running out? I've never seen it.
But for the sake of the discussion, I will imagine finding myself in the place of the couple. I get to the table and see a bunch of empty dishes -- no food left. I'd absolutely say something to the host. I was invited over for dinner, I brought my potluck dish, yet no food for me. I'd probably just say something like "oops, Ms. Hostess, looks like there's nothing left for me to make a plate from." At the very minimum I imagine she could offer me something from the kitchen.
I think it's pretty PA to just act like nothing is wrong at all and leave early without saying why. As hostess, if someone is uncomfortable or inconvenienced or [horrors] unfed! at a party at my house, I sure want to know about it so I can at least try to remedy the situation. But as I said, in this case, I'd be in the same boat with the hungry guests who would have been in front of me in line for the food.
And thinking about this some more, I can't imagine, as hostess, not realizing that there wasn't going to be enough to go around before the first person even got in line. You know how many guests you have, you know what food has been brought. If the food appeared to be lacking, I'd probably break out some extra stuff, cheese & crackers even, to round out the selection.
I agree with posters who find the fact that the exact thing happened twice to the same couple a little hard to swallow. But you know, you have to take those letters with a grain of salt. In order to make them more interesting and yet concise, I imagine there's a certain amount of editing that goes on before the letter actually gets printed.