The question always is not the information itself, but why is this being divulged now and who is benefiting from its disclosure?

This isn't just "faster than the speed of sound" like supersonic jets. This is hypersonic. We are talking Mach 5 and above. At the moment this requires scramjet propulsion to sustain these speeds. Scramjets also can't operate at low speeds so you normally require a multi stage system with jet engines or boosters as well to launch and reach operating speed. This technology, in the public domain, is immature.

Furthermore you encounter some knarly aerodynamic issues in the form of some bizarre shock formations and severe structural issues due to the extreme temperatures. All up it's a very difficult multi-discipline optimization problem.

Missiles traveling at hypersonic speeds are an issue because it's near impossible to deploy countermeasures in time to stop them.

However, yes you are right. There is no way they didn't know. This is a huge area of research right now and a priority threat to every nation on the planet.

Source - me, I do this for a living.

Edit: I almost forgot the most important part. These are not ICBMs. ICBMs are intercontinental ballistic missiles. These hypersonic missiles are not ballistic. They are capable of powered and sustained flight. This means they are capable of more advanced maneuvers, can traverse longer distances, and can loiter above a target if necessary. Their ability to alter the flight trajectory mid flight makes them unpredictable.