Thunderstorm Land is just a big cocoon, with not so strong constraints, otherwise others wouldn't be able to get in. It's not that difficult for Zheng Yichen to leave Thunderstorm Land.
The reason he can't leave normally is not only due to Tia's influence but also his own reasons. Tia used this reason to influence Zheng Yichen.
A too weak self makes Zheng Yichen subconsciously worry that he might bring greater destruction to the world, and this destruction would affect the relatives remaining in the New World.
Tia strengthened Zheng Yichen's self-restraint, making it harder for him to access outside information. In other words, she used this to 'monopolize' Zheng Yichen.
But if Zheng Yichen really wanted to leave here and had the means to do so, Tia wouldn't be able to stop him. If Thunderstorm Land ceased to exist, how could it obstruct him?
