I am not going to invoke the K-word in this question!
Hypersonic long-range craft have to fly pretty high to travel circa mach 20. Would they need to travel into space to do so efficiently?
ICBM’s don’t typically fly at hypersonic speeds for the entire duration of their flight. They do typically cruise at altitudes above 100km to take advantage of reduced drag, but they do so without engines turned on. They follow suborbital trajectories with velocities that decrease up to apoapsis and increase upon reentry. This particular missile (avangard hypersonic missile) uses a supersonic combustion ramjet engine (scramjet), which would not work in high knudsen flow (rarefied gas dynamics). So while it won’t accelerate while in space, it can accelerate to suborbital trajectories that traverse space.
Cruise Missiles, on the other hand, usually stay well below 100km in altitude. But they don’t tend to travel at hypersonic speeds either. It would be interesting to know the actual altitude that the missile flies at, but I suspect there may be a strategic reason for not disclosing it publicly.