The waitlist is real, but the waitlist is huge.
I’d guess something like 50% of applicants end up on the waitlist at most schools. It’s everyone who’s perfectly qualified, but didn’t make the cut for whatever reason.
For example, in my DS’s K class, almost every boy coming in K was a sibling. Odds of them turning down a spot is slim. So the waitlist was “real” and there was absolutely nothing the wait list kids could have done differently to get in, but there were only a couple non-sibling kids that year and odds of waitlist movement were incredibly low. If someone HAD declined a spot, they’d look at the large pool of waitlist candidates to fill that one spot with someone similar-ish.