Now the demon king, in his cave, said gleefully: ‘Little ones, Sun the Pilgrim has come off badly and gone.

The scene cuts to the cave and the demon in high spirits. 吃了虧去了 -- came off badly and left -- is the same phrase Wukong used about himself on waking, now in the mouth of the person who did it to him.

Though I could not kill him this round, at least I gave him a great swoon.

An honest self-assessment, which is what makes this demon dangerous. He does not claim to have killed Wukong. He says he could not, and settles for having knocked him senseless. Monsters in this novel usually overestimate their victories; this one does the arithmetic.

But wait —

咦 yí is a noise of sudden realisation -- the sound of a thought arriving. Not the cry of grief that appeared when Wukong's body came down the stream, which is a different character with a different feeling. This one is closer to "hold on."

I fear he may go and fetch reinforcements again.

And the thought is the right one: he will go for reinforcements again. The demon has correctly predicted the pilgrims' next move while they were still deciding it.

Quick, open the gate, and let me go see whom he is fetching.’

So he goes to look. Not to attack, not to celebrate -- to gather information. This is the single most competent thing anyone does in the chapter.