This leaves me wondering what happened to the information they got from Pershing II RVs?
of data. But there's lots of different kinds of data to gather too. There is all kinds of hypersonic research, from how to control a hypersonic vehicle in flight (which you can do by firing an HV on a rocket and then letting it glide at hypersonic speeds, like they did with the HVX-2) to the major challenge, which is an air-breathing hypersonic engine.
I haven't looked closely at DARPA's XS-1 program, but off the top of my head my guess is that gathering hypersonic data is just a box that they checked to get that community on board. It is common with research proposals (for instance, when people are asking for money) to claim that your project will not only reveal the secrets of the universe, but will also cure cancer and work well as a floor polish. So my guess is that they looked at the regime that this thing will operate in and said "Hey, let's try and get the support of the hypersonics people too, because it will fly hypersonically."
The U.S. made substantial progress on hypersonics in the past decade or so, but there are still some big gaps in the data. There is no concerted U.S. hypersonics research program right now (it has sort of fallen through the cracks), but at least China is still going strong!