If a modern ICBM can travel at speeds of 6-7 km/s (around 25000 km/h) and the speed of sound is 1192 km/h that's by my math Mach 21. So if a modern ICBM can travel at Mach 21, why are new hypersonic russian weapons capable of surpassing west's defence capabilities if they travel at mach 5-15 ?
I found that "hypersonic" is speeds of mach 5 to 10, so AFAIU the west already has hypersonic ICBMs for a long time.
Hypersonic weapons aren't meant to defeat ICBMs, they are designed to defeat air defence systems.
Current air defence systems are designed to work against fighter aircraft and their missiles, which travel at speeds of Mach 2-5. Air defence missiles also travel at Mach 2-5 because that's fast enough to defeat those threats. A Mach 15 threat reduces the reaction time, and makes the missile much harder to hit (esp if your defence missiles are slow).
Some work has been done in defence against ballistic missiles, with pretty good results against old Scud theater ballistic missiles. ICBMs are much faster and harder to hit than those, but because of their ballistic trajectory (which gets them to very high altitudes) you can at least see them coming.
Hypersonic missiles are the next step: they combine speeds almost as high as the ICBM with a much flatter trajectory, which means they become visible later which shortens your reaction time.
ICBMs are very different in nature and purpose from the hypersonic cruise missiles described in that article.
Medium-range or intercontinental-range ballistic missiles travel in high ballistic arcs -- usually leaving the atmosphere and reentering. Since they go up high, they're relatively easy to detect while they're still 10+ minutes away from the target, but interception is still difficult due to the use of maneuverable reentry vehicles and decoys, and the high speed of reentry. Their tactical use is limited, because their use is likely to trigger a strategic nuclear response, so they would only be used as part of a general nuclear strike.
Hypersonic cruise missiles, in contrast, fly at relatively low altitude, and so cannot be detected at the long ranges that ICBMs can be. They are designed to reach tactical targets -- coastal military sites or high-value naval targets -- and can carry either conventional or nuclear warheads. At mach 8, if you detect a missile at 160km, you have only about one minute to try and stop it. The US is very concerned about such threats to its aircraft carriers in particular.
The glide missiles mentioned in that article are sort of a hybrid between ballistic and cruise missiles. Like an ICBM they are boosted to high-altitude, but then a winged stage much like a cruise missile takes over, which allows the missile to steer itself in atmosphere more drastically than a conventional ICBM warhead; the only real advantage there is that the final destination of the warhead can be kept secret for longer.
