Americans apply AFTER graduating from high school??

For American colleges, you apply in the middle of your senior year in high school, as colleges don’t expect you to have a diploma already. But if I understood correctly, McGill requires that you already have a HS diploma while applying? How does that work? So if I graduate HS in 2016, I would then have to apply to McGill to enter in 2017? That sounds like a lot of waiting. In the U.S, we enter college the same year we graduate from high school.
Here’s the link: http://www.mcgill.ca/applying/admissionsguide/2015-16/usa#DEADLINES
Can someone clarify? Any guidance is appreciated.

You apply during your senior year like a normal US College. However if you flunk out/don’t graduate, then you can have your admission rescinded.