Honestly many schools have this, and with far less transparency about the number of spots. Some have ranked waitlists, some have unranked waitlists. If you've ever seen someone say that they've been placed on a pre-interview hold, that's what this sounds like. We've already been given a lot of information that most schools do not give out. I can't imagine getting much more information about the waitlist would be helpful. Yes, there are probably very qualified people on the waitlist, people who may be just as qualified as those getting interviewed, additionally there are probably very qualified people getting rejected. This school has 80 seats, not getting interviewed here does not mean that the other 5,000 people that applied were completely unqualified or poor candidates. Knowing whether or not you made the 10 person waitlist cut isn't going to help you improve your app in any meaningful way if you don't get an II here. If that happens, asking for feedback and taking a critical eye to your app would be far more beneficial. Keep in mind too that schools are trying to build diverse classes, the difference between someone being waitlisted and interviewed could be something like "well we already have X many people with research heavy backgrounds" which wouldn't be useful information for you to have, you can't help it if that's how you spent your time. I'm not an adcom so I might be dead wrong and I'm not trying to be harsh, but I don't think you'll get any more information about the waitlist or the criteria.
There is probably a finite interview amount. They only have one more day scheduled. They do MMIs. If they interview 30+ people a day, that means that are a good number of people who need to be pulled from university business to go spend a day doing interviews. That takes time and money. To just add more spots you would need to find people willing and able to participate in the MMI process on short notice. I work in a lab at UA-P and my PI has participated in the MMI process before, and we did not get much done that day because of it. It would be great if they could interview more people, but I definitely see why they cannot.
All that said, good luck to you! Maybe you are one of the lucky waitlisters, and maybe someone is turning down a spot for you as I type this. 🙂