Way to think of it is when it’s closed at the end, it’s all good.
You don’t need to think too much about having closed parts while editing to have a valid solid to print out. Unless, the parts are a separate piece. (In this case not)
The naked edge in your sample is coming from only parts of edges of a surface connecting to parts of edges of a surface. Polysurface looks connected, but it’s just held together only by few edges.
If you use extractsrf, to extract the hinge part one piece at a time, you’ll can see that
naked edge is partially there in the main watch part.
If you explode then join the main part again, it’ll remove the naked edges.
So when you joined the first time rhino tried to join everything as much as possible.