I don’t get how the incoming storm is going to make things any better. It’ll dilute the soup in the big pot, stretching how many it will feed? It will scare away the less hardy soldiers, so our guys will move up in line right away?
Or could he mean, Relax, we’re not going to even get close?
Comics I Don't Understand 
I think “rain watering down the soup so it will last” is probably the best answer.
This one is perplexing for sure. It could be that the soup will be diluted and stretched out, but who wants watered down soup? And the cook could just add water if that’s the standard method for ensuring everyone gets to eat. I probably go with the storm scaring away some of the soldiers. But a lightning bolt would make that argument more convincing. I can’t imagine soldiers being afraid of rain.
I think Beetle is being sarcastic. The rain will dilute the soup. Which is probably already too watery. And in a few minutes, we will have other things to make us miserable.
Agree with beckoningchasm; hard to believe military being scared away from food by rain.