I would consider leg press, squat and bench press to be closed kinetic chain exercises as there is weight applied to that distal segment. I tend to think of open as exercises where the joint is not being used in the normal way it would to support it. A squat or leg press will use gluteals, hamstrings, quads, ad/adductors, calf muscles etc during the exercise. A knee extension however uses mainly the quads in an abnormal or more 'non-functional' movement of the knee and I think of it as a more open chain exercise.

You are correct in that the definitions are confusing and not well thought out in terms of what is relevant. In general think about normal movement in exercises when trying to rehabilitate an upper or lower limb biomechanics or strengthen support around a joint e.g the knee of shoulder. Use open perhaps at times where there is a specific weakness in a specific muscle that needs strengthening ahead of performing closed chain exercise.