Very true, but it's like that through the entire cycle. Most MD schools, and certainly most top schools, simply do not need to waste time and resources interviewing for a WL.
Schools like WashU and NYU just like having a lot of candidates to choose from. Other schools like Michigan and UVA do the screening on the front end and have very high post-II acceptance rates. None of these schools, however, lose track of how many seats they have and then just interview for the WL when they run out.
As I said in another post, this in contrast to DO schools, which intentionally fill their classes early with people they KNOW (or hope) they are going to lose later to MD schools. They do this to collect significant, nonrefundable deposits that are not part of the MD school revenue model but contribute significantly to DO schools. As a result, DO schools do actually start interviewing for the WL around now, but they typically make extensive use of those WLs in a way MD schools don't, because MD schools are not in the business of accepting people they expect, or even hope, to lose later in the cycle in order to generate revenue for the school.
Apples and oranges. As you said, the vast majority of everyone interviewing at WashU will not be attending, either by choice, since their yield is on the low end for a top school (being in St. Louis probably doesn't help when top candidates are choosing between NYC, Boston, Philly, California, etc. and St. Louis) and because they interview over 1,000 people to make less than 400 offers. (It's actually not 19%, because that probably takes the low yield into account.)
According to my numbers from a few years ago, they accepted 375, including people coming off the WL, to fill 124 seats, including 20 MD/PhD.) Either way, people are no more interviewing for the WL in January or February than they are in September or October. It's just brutally competitive to get into any school, let alone a T10(11). Really doesn't matter when you interview. If they are taking the time to talk to you, they have an available spot to fill. You'll see -- people interviewing now will later report receiving As without making pit stops on the WL.