The HGVs will be boosted into space from deep inside Russian or Chinese territory—beyond the reach of missile defenses. We would need a space-based platform to take them out, or send platforms like the B-2 or B-21 trolling over vast stretches of territory to take them out before launch.
Don’t be so certain we’re “leading in hypersonics” in general. This has not been a priority for the Obama Administration (imagine that) and our operational tests have been failures.
Aerojet-Rocketdyne has made considerable advances, and their are others.