That vehicle is a sub-orbital vehicle with a re-entry trajectory similar to a ballistic missile . It reaches it's highest speeds only at altitudes where the atmosphere is more individual particles than a continuous gas (it's apogee of 62 miles is well above the Karmen line). The definition of Mach number only makes sense within a fluid. A vehicle that travels 20,716 miles per hour is NOT travelling Mach 27. You can say it's travelling roughly 27 times the speed of sound at sea-level like conditions to get a sense of the scale of the speed, but it is not travelling Mach 27. Once it slams into the atmosphere on re-entry, you can say it is travelling at Mach 27, but by then it's not undergoing sustained flight. This specific vehicle is merely gliding.

This distinction is important, because all of the difficulties associated with flying at hypersonic speeds are due to the problems associated with flying through the fluid. It's comparatively easy to accelerate to the high speeds of sub-orbital vehicles once outside a meaningful atmosphere.

Militarily this is an important distinction as well. An intercontinental ballistic missile needs to go significantly above the earth's atmosphere. During this phase, it's easily detectable, since the horizon is much further away. A vehicle flying much lower well within the earth's atmosphere at very high speeds gives much less early warning.

This specific vehicle strikes a sort of middle ground between these concepts. It's half sub-orbital ballistic missile (it's still required to go above a meaningful atmosphere), half hypersonic cruise missile. Although very critically it is not undergoing sustained flight at hypersonic speeds

Air breathing hypersonic missiles and hypersonic glide vehicles really shouldn't be compared. They have different operational uses. Air breathing missiles are slower but can also be significantly smaller and can fly for sustained periods at lower altitudes. This missile is more comparable to the Russian Zircon missile.