Ummm, I think what we have here is something a *little* bigger and more complex than teenage miscommunication. Beyond the privacy that the Seonho group would expect ie for Ido to not blab to a former exec and *son* of the Haesin CEO about an aggressive takeover, it's obvious that Seijin still feels a deep loyalty to his family. He came home with signs of physical abuse and insisted that "I fell". He knew of at least some of the problems within the company already but learning of the high levels of wage theft (!!!) and embezzlement addition to the laundry list of crimes only has him concerned about his shitty rich family.
It's incredibly hard for an abuse victim to trust anyone and Ido was and is in a position where he had to keep his mouth shut. This is complicated by the fact that they're into each other. I think the author has done a good job of creating genuine conflict.
As much as I'm a fan of these types of Mangas like I'm so over not giving explanations or the miscommunication like TF are teenagers that we can't tell each other that's going on how feel what our plan is...but whatever....