Too fast to intercept? I doubt that. What was it, a couple of years ago the US shot down that satellite to prevent Hydrazine from reaching the ground? Well, that was the story, right? It also demonstrated that we could hit something travelling at orbital speeds. This system is sub-orbital, starting at a lower velocity. Yes it can maneuver - to some extent. Every maneuver scrubs off speed and energy. Ultimately it dives towards a target, not significantly different from other "standard" ICBMs. Terminal intercept systems shouldn't see it as being much different from anything else.
But that's just technical fun. Big picture, even if it works as designed (a big if with new tech) and even if it cannot be intercepted - BFD most of Russia's existing ICBMs cannot be intercepted. Oh sure, we can probably knock down the first 30 or 40. Russia has nearly 300 ICBMs. So at best we get what, maybe 10%...
The US missile defense system isn't designed to counter a massive doomsday strike. It is supposed to stop terrorists and nation-states such as Iran and North Korea. Iran and NK are far more likely to use their weapons than Russia or China. We don't see eye to eye with either Russia or China, but they are at least nominally sane and rational by our standards. They "get" mutually assured destruction. However, Iran and their cultists...not so much. They just might go for it, even knowing they'll become a glass-topped self-lighting parking lot, figuring their reward is waiting in paradise. Ditto NK, so power-mad that if they see it all coming apart, they'll probably launch on us, Japan, and SK just out of frustration and petty BS. I don't trust the leadership of either of those countries.