in ch.23, when the 2 men congratulate each other, what are they congratulating each other for? How is that ironic?

I think this happens at the very end of Chapter 23,

"There was some long grass. The youth nestled in it and rested, making a convenient rail support the flag. His friend, jubilant and glorified, holding his treasure with vanity, came to him there. They sat side by side and congratulated each other."

The congratulations seems ironic because it is a battle they could have just as easily lost. The last "Grey Coats" they took prisoner simply didn't hear their comrades retreating. This is the only reason they stayed to fight.