IIRC the Kinzha is only hypersonic during boost. After that it is continually slowing. It is supposed to be a maneuvering glide vehicle. Same thing with the Chinese hypersonic. Only during boost.
I'm curious to learn more about how Kinzal and Chinese hypersonic missiles are intended to work, but there is a dearth of detailed public records on that subject.
Do you have any sources on Kinzal flight profile that you could share?
From my understanding, the difference between classic basilic missiles (which can and do reach hypersonic speeds) and "new" hypersonic missiles is that the latter fly lower, inside the atmosphere, and use air to change trajectory in flight. Thus, they can fly on a less predictable and lower trajectory path, which is harder to detect and provides lower reaction time to intercept.
From that perspective, it makes little sense for Kinzal (or Chinese hypersonic glide missile) to slow down in the final phase of flight. Once accelerated to Mach 5-10, some kinetic energy dissipation will happen during maneuvering, but it would not make much sense to shed so much of it as to drop into sub-hypersonic flight speeds.
What is more likely is that
Kinzal, just like the Iskander that it is based on, is a ballistic missile with limited course correction capability, that can reach hypersonic speeds. More easily so by Kinzal due Migs effectively providing the first stage of acceleration for the missile.
Chinese hypersonic glide vehicle HGV missile, on the other hand, is boosted by one of Chinese ballistic missiles to Mach 5-10 speeds, then glides through atmosphere at hypersonic speeds.
Unless, of course, Chinese are lying as much as the Russians!
Either way, it's very impressive that Patriot battery managed to intercept 6 out of 6 Kinzals that were fired at it!