There are defectors, yeah. Most of them by the way just leave the country to China through the normal channels and then don’t go back, which kind of discredits the idea that there’s no way to leave the country in the first place.

The ones who go to South Korea are generally held in prisons by the South Korean government and interrogated until they’re determined not to be spies, then treated as second class citizens by South Korean society. There is an industry around putting them on TV so they can talk about how horrible the north is, with clear incentive to exaggerate or invent claims as only the most exaggerated claims can keep you in the spotlight and possibly get you a big payday and an option other than being a second class citizen slaving away in poverty. It’s also illegal in South Korea to speak positively of the North or of Kim.

There are also cases of people who defected from North Korea to South Korea and later decided they would rather return to the North. This is forbidden by the South Korea government, those people ate not granted passports and are barred from ever leaving the country. You can watch a documentary about these cases called the loyal citizens of Pyongyang in Seoul. https://youtu.be/3V4Hnl7J9H4?si=J5VTemHyz-EvEja5

Idk which claims you think I “don’t believe” by these defectors but some are patently ridiculous like much of what Yeonmi Park has said, other claims are things that are probably true but again, difficult conditions in North Korea are largely the result of the U.S’ near genocidal war and sanctions regime against them in the first place, and much of what could be described as “authoritarian” is a response to the existential crisis imposed on them by said war and sanctions regime, and is necessary to prevent the destruction of their state (which it should be noted has far more legitimate origins than the South Korean state, itself established as a military dictatorship and puppet of the U.S.)