Although Ling Tian knew that Po Qiong cared somewhat about Hua Min'er and the others, he also knew that Po Qiong wasn't someone who placed much importance on parting and death. In other words, Po Qiong didn't care whether Ling Tian saw Hua Min'er and the others again before breaking free of the Lord of the Universe's control—especially since they couldn't communicate with Ling Tian. So if Po Qiong insisted that Ling Tian enter the Outer Realm Stone, there had to be something else he wanted to tell him, something the Lord of the Universe must not know.
Based on his earlier guess that Po Qiong had asked him to improve the many grand formations for a reason, Ling Tian was even more certain that Po Qiong had something important to tell him, something the Lord of the Universe must not know.
In the God Realm, Ling Tian and the others had no secrets from the Lord of the Universe. Even if Po Qiong spoke to Ling Tian through voice transmission, the Lord of the Universe might still overhear them. The only place where they might be able to hide something from him was inside the Outer Realm Stone.
It made sense. The Outer Realm Stone had been left behind when an Outer Realm Cultivator—a tremendously powerful one—perished. It contained an immensely vast and pure supply of High-level Outer Realm Power, a force the Lord of the Universe couldn't control. It could even greatly reduce the erosion of the power of time. If Ling Tian hid inside the Outer Realm Stone to speak with Po Qiong, the Lord of the Universe might not hear them. Then Ling Tian would know things the Lord of the Universe didn't—and perhaps those things would help him when he faced his tribulation and tried