Its because they havent really shown us the complexity of his character yet
The story has been planting pieces of his personality from the beginning. Mokyeon points out that Nanjo has a smug sense of superiority he tries to hide, the author's note says he hates losing, and even after everything that's happened, he still frames the situation as "I let things get out of control" instead of admitting he completely lost control. That tells you a lot about his mindset. At the same time, he's also someone who believed the person he loved was being abused, sacrificed himself to save him, endured repeated coercion and humiliation, and eventually snapped under all that pressure. Those traits don't contradict each other but they coexist. That's what makes him complex. People seem to think a character can only be either a victim or arrogant, when Nanjo is clearly written as both. That's nuance, and the story has been showing it instead of spelling it out.
**Okay not loved but liked it (the twink)
People can understand Mohk because he's a complex character, but when it comes to Nanjo they suddenly become dumb and go, "I don't think people can have that personality." Like... have y'all become so used to MCs being either doormats, sassy, or tsundere that you've forgotten people are actually complex? Not every character has to fit into the same handful of personality archetypes. Different people have different behaviors, coping mechanisms, and ways of expressing themselves that's what makes them feel human.