This is about developing a plane that can fly at Mach 20 (and if my math hasn't abandoned me, that is roughly 15,300 mile per hour). That causes all kinds of problems with control surfaces.
There are several serious risks to vehicles that achieve extreme hypersonic flight. A vehicle flying at Mach 20 will experience intense heat (more than 3,500 degrees Fahrenheit) and extreme pressure on the aeroshell. According to DARPA, 3,500 degrees Fahrenheit is hotter than a blast furnace that melts steel. The IH program hopes to discover more about what thermal protection materials and hot structures technologies are needed to withstand the intense temperatures created by hypersonic flight.The goal is to be anywhere in an hour. With the world's circumference being about 22,000 miles (depending on starting point and path followed, 15,000 mph gets you there with time to spare.)
Still the fact that NASA had a decades-long research program into SCRAM jet technology and shelved the project as soon as they were successful was probably a broad hint that this program existed. (NASA only got to about 5 to 7000 mph. And they still had problems.) But it just doesn't seem that a properly paranoid alphabet-soup-agency would let this kind of information be common knowledge. CIA, NSA, DIA, whatever. You would think they would want this kept low-key.