Can't a radar differentiate between stationary return and that from a hypersonic object ???
Depends on the data processing.
Ever play 'chicken' ? That is when you and the other guy are on a head on collision course and the loser is whoever breaks the course out of fear. But while both of you are in that course, from each person's perspective, neither one of you are moving, at least laterally.
Likewise for the radar, if it is programmed to process only angular displacement, then even though the missile is approaching in a head on collision course, regardless of speed, the radar will see the missile as a 'stationary' body. No defense radar will be this limited, of course, because the intention is to intercept the attacker from as far away as possible. So in the event that the radar and the attacker is in a perfect head on collision course, how is the radar to calculate distance ? Via the Doppler component.
Doppler processing is an option in every radar system design. However, because military radar systems have to be as versatile as possible due to nature of threats, Doppler processing is pretty much standard built-in. So in the event that a radar and a body just happened to be in perfect alignment in a head on collision course, with no angular displacement echoes for the radar to tell if the body is moving or not, Doppler component processing is the last resort. As the missile and the radar approaches each other, Doppler frequency will increases which equals to decreasing distance, which equals to response time, or rather shortened response time.
This is why low horizon and high speed approaches are so difficult for the defenders. Not impossible, just extremely difficult.