This is patently false. You drink at the nomikai because everyone drinks at the nomikai. If you don't join, then you are seen as not being a team player. It's as simple as that. There is no subterfuge going on. Hell, the boss is usually the one that ends up the most drunk. Yes, perhaps some people may use it as a way to gain some insight, but you can apply that to anything.

And in addition, whatever is said or done at the nomikai stays in the nomikai. It's a way for the team to blow off some steam and be honest with each other. The next day, people carry on as if it never happened. I've been the drunk guy and the sober guy in these situations. You do learn a lot, but it's not the purpose of the gathering.

Source: I've lived and worked in Japan and Korea for over a decade.