Well mach 10 for Tzirkon is at high altitude. Ships are a height of 0m. So the missile will have to go through high density atmosphere where it will loose speed.
It will be flying at an altitude of between 30km and 50km and as it approaches the location of its target will most likely pitch up and do a short climb and then dive down nearly vertically on its target in full thrust mode... and as it will have burned off most of its fuel weight its power to weight ratio in its vertical dive will be impressive... moving at mach 10 it will be descending at just over 3km per second so only the last three or four seconds will the air actually be very thick, but as I said it wont be free falling like a bomb... it will be coming down with the equivalent of a rocket engine propelling its descent...
Rather than slow down I would expect it would speed up...
This data would be valuable for US navy planers. Because if the speed fall to mach 4 or 5 it is still possible to intercept it.
The idea behind the scramjet is that it can manouver and not lose speed... I would think it would not lose speed when heading down to the target either...
In any case, while US defenses might be able to intercept M4-5 missiles, their ship-based ABMs are designed to intercept at range, and won't be much use in terminal attack phase at short ranges within a few kms.
Another factor is lead... when firing a cannon or gun at an incoming missile you lead the incoming target so in the few seconds it takes for the incoming threat to be reached by your cannon round or missile the target threat will have moved.
At mach 10 in a dive that means in one seconds time the target will move 3km, so a cannon or missile fired three seconds before impact that is 2.5km away on another ship and moves at mach three would be aimed at the ship if you launch/fire it when the incoming threat is at 9km altitude... and into the water below the ship if fired any later...
The quoted section forgets about the speed difference of TLAMs and hypersonic missiles. There is not going to be a perfect response from the American ships.
Some will launch TLAMs soon enough, others will not be so lucky.
A loose association of Soviet and Russian air defence vehicles not integrated in an air defence network and positioned around potential targets was able to shoot down 73 out of 103 missiles fired at targets in Syria... every missile that was fired upon was shot down... many of the missiles not shot down flew into the ground because of EW. The targets that were struck were empty and not actually defended by anything on the ground because the intel was bad.
I would honestly say that 16 missiles would only be enough for a very limited range of targets most of the time... a single vehicle like a TOR could deal with four targets at a time and so could a single Pantsir system.... a full land based battery of TOR or Pantsir operating normally as part of an IADS could easily defeat an attack from 16 subsonic low flying missiles.
In fact the new four missiles per tube anti drone missiles for Pantsir would probably be enough to get a kill per missile and their very high speed means even a single vehicle could probably start engaging targets at 20km and engage more than a waves of missiles at once.
With missile flight times to 20km of about 20-30 seconds the first wave of four missiles being engaged at 20km could be defeated at 20-18km range so the next four missiles could be engaged at 16-14km range, and another four missiles at 12-10km range, so 12 tomahawks are likely dead before getting within 10km of the defence vehicle... leaving four missiles and a Pantsir vehicle with no missiles left, but with guns... but Russian vehicles operate in batteries of 6 or more vehicles so while that vehicle was shooting down 12 missiles the other 3 to 5 vehicles could easily take down 36 to 60 more targets... and that might not be the battery protecting the target the Tomahawks were aimed at... it might have just been a Pantsir battery on the way to the target area...
TOR is worse because although it has a shorter range of 15km, it carries more ready to fire missiles with 16 ready to launch missiles per system and the improvements in accuracy and radar means they no longer fire two missiles at each target...
Russian and Soviet air defences were designed to defeat subsonic threats and supersonic threats too.... hypersonic threats are new.
The west is hoping numbers can overwhelm when it can't deal with speed... Russia had speed but lacked numbers but now has numbers and speed and HATO is not well defended against either.