Dude you have no idea what you are talking about. So leg extensions and leg curls are the only open chain exercises for legs? There is no difference in the definitions, you just have no clue what you are talking about. No wonder you think it is dumb you dont even have the concept down.
Closed Chain refers to closed kinetic chain. The kinetic chain of a leg press is not closed because you can push the weight period, that end of the chain is not closed, it is moving. When squatting even if doing jump squats, when you press your legs into the floor the kinetic chain is closed on that end. Leg Press, Lat Pulldown, Bicep Curl, all these are open chain.
There is basically no limit to where the kinetic chain can end, thus it is open. The floor is not going anywhere. The same as a pullup bar. These are closed.
To answer the other posters question, I dont know about the closed chain leg press being better or not. I dont think that is the take home idea, as long as you can manipulate an exercise to be closed chain it is better. It may be a bit better somehow but it is definitely not as good as bodyweight exercises.