After the particle world was sealed, he clearly didn't wish for these immortals to leave just like that. If this was the was the case, no one in the immortal realms would know the things that happened in this particle world. Not even the Eastern Sage Immortal Emperor.
Although even if they knew what happened in here, they would not necessarily be able to guess that he has an avatar. However, Qin Wentian had always been cautious when he handled things, not keen on taking unneeded risks. As for these immortals, many of them did as they desired, becoming despots or tyrants in his particle world. It was better to make them stay here by sealing their cultivation bases. It could be considered a punishment of sorts.