I just got back from Whitney and like you I was very confused about the walk in campsites so let me save you some time because I just finished going through it and I can help you not make the same mistakes.
Thanks, this was super helpful! I found the "secret" Ravine Campground exactly as you described, first come first served, and there was literally nobody else in the whole campground except us. The way it's set up, this campground seems almost intentionally hidden, invisible from the road, and there's no signage and only a faint trail leading into the trees. Beautiful location, away from all the noise and traffic, which made it a lot easier to get some sleep.
