According to Ling Tian's analysis, he had once come to the Outer Realm through the opening in Shi Tian's Small World. He simply hadn't known it at the time.
Now, Ling Tian's clones were still in Shi Tian's Small World. Though they had no golden cores, after all this time, even advances in their Nascent Soul cultivation would have greatly increased their strength—at the very least, far beyond what it had been when Ling Tian was in the Cultivation World. They could therefore undo the seal on the opening and come to the Outer Realm once more. If that happened, Ling Tian would have a chance to sense them through certain means, greatly increasing his chances of breaking through the firmament.
If he broke through the firmament through his own strength, rather than because the Lord of the Universe deliberately let him win, then that passage would undoubtedly be under his control. In that case, he would have an even greater chance of taking Hua Min'er and all his other friends and family out of the universe. The thought filled Ling Tian with delight.
Still, Ling Tian didn't act rashly. It wasn't just that it would be dangerous for his clones to leave the God Realm and come to the Outer Realm. More importantly, Ling Tian had yet to reach his peak, and he wasn't confident he could break through the firmament yet. He could only be patient and wait for the right moment. There were powerful cultivators in the Outer Realm, and they could pose a tremendous threat to his clones. The longer Ling Tian's clones stayed there, the greater the danger.
Besides, the Lord of the Universe controlled a vast region of space. If he saw Ling Tian's clones leave, he might capture them and throw them back