Actually, the Bull Demon King did not finish his sentence. What he wanted to say was that if he could be taken away from Flame Mountain, he would be free from control. However, before he could say the rest, all his consciousness was suppressed by Tathagata. In fact, Tathagata is not omnipotent; the reason he could manipulate the Bull Demon King from the Western Paradise Sacred Mountain is that there is a golden statue of the Buddha of Tathagata within the eight hundred miles of Flame Mountain.
Once the Bull Demon King leaves the boundaries of Flame Mountain, that golden statue would no longer have an effect on him. Although the Bull Demon King didn't finish his words, Sun Wukong still understood his intent. Thus, he deliberately retreated, luring the Bull Demon King to chase him. As expected, the Bull Demon King, controlled by Tathagata, immediately pursued him.
