If the email simply said "Free lunch, in the conference room, now!" I guess technically, the other employees weren't rude by taking the food.
BUT - it also seems (and correct me if I'm wrong, TootsNYC) that:
1. The other employees knew full well that your team was under pressure, and didn't have time to step outside to get lunch;
2. The amount of food was limited. Ie, there wouldn't be enough for all staff; and
3. This had been going on for at least a week.
In that case, I do think the other employees were somewhat inconsiderate for rushing over to grab "first dibs" on the food.
As for a solution, I wonder if it would have been rude, after Food Guy send out the initial email, to quickly send another email to everyone saying "Thanks Boss. My team and I are working really hard, and don't have the opportunity to step outside. Lunch in the conference room sounds great. We'll be there in
That gives everyone the heads up that your team intends to have some of the food. And hopefully the other staff will back off, and leave enough for your team.