Me personally? I'm sure it happened...in some sense of the word.
But lets assume for a minute that this happened as she said.
If it was the same host twice, then yes, speak up! The host needs to know that some guests aren't being fed and find out why. If people are taking outrageous portions, that's something that can be controlled by just providing smaller plates or reminding people that there's enough food to feed 40 here, so make sure to leave enough for everyone. If people aren't bringing enough to begin with, ask people to prepare bigger sizes, or bring 2 dishes. If not everyone is bringing something but still shows up and pigs out, make a sign up sheet... all of these issues can be avoided.
If you're just too scared to get to the table and feed yourself though...well...that's no ones fault but yours.
Unless this was the same host twice, I just can't see it being factually true.
If it was about plates... ask the host if she has any more before running away. And I don't know how it could realistically be the "overly aggressive" guests fault that the plates ran out unless they were juggling them or using them as frisbees all willy nilly.
Honestly I doubt that the situation was accurately described here though. Why, on two different occasions, would you bring gifts for a hostess? Maybe that's an etiquette rule I'm unaware of, but it seems odd to me that you'd do that if it wasn't for a birthday or anniversary or something. How would only this couple be left without food...twice? If it's plates, why wouldn't you ask for more before running off?
It sounds more like they didn't really want to be at a pot luck, so they made an excuse to leave and didn't want to tell the host the real reason.