Inverse square law, at 20 miles the effect is low. A 200kt warhead at 100m is more effective than 5Mt at a couple of miles.
ICBMs are hypersonic, the question is do suborbital air breathing missiles provide any advantage.
If you don’t intend to intercept MIRV warheads (and the USA don’t, their interceptors are for the odd rogue launch) then launch detection is enough to preserve a response to first strike; and the reported missile needs an IRBM/ICBM launcher to reach SCRAM engine ignition.
The question to ask is whether this weapon opens a defence hole which needs closing, and I don’t think it does.
Just like the Russian submarine nukes being boasted of by Putin, where’s the added value? MAD is MAD.