No-one is actually planning on nuking anyone. But having a missile that can't be intercepted is a solid '+1' in the nuke arms tally.
I agree with you.
It's not undetectable as the American SBIRS system would detect the IR signature of any launch. So, we'd know where it came from.
What this type of system provides is twofold IMO:
First, unlike an ICBM which travels at high apogee and arcs deep into space onto it's target in the shortest path, this uses a Fractional Orbital Bombardment System (FOBS) that shoots the vehicle into low-earth orbit -- in any direction -- where it can circle the earth a whole bunch of times and then come down on it's target. So, it can circle the earth for days or weeks then come down. And, more importantly, the American anti-ballistic missile shield is designed to stop things from Asia which arc over the North Pole. Thats why the X-band radar and interceptors are in Alaska and California. With FOBS, the Chinese can launch south, have it orbit a few times then come down on the US from the south, where we have limited radar coverage and no interception ability in space. This has been around since the 1960s.
Secondly, the hypersonic vehicle provides a new spin on the FOBS concept as it grants the ability to get around terminal defenses the US has like THAAD or PAC-3. It's moving so fucking fast and can maneuver, it's unlikely the hit-to-kill stuff we have could lead to an intercept either in the atmosphere or outside.
In my opinion, and I'm not an expert outside of some work done on Cold War military history at SAIS in my younger days. This is kind of a blustery bullshit weapon system at this point. It doesn't change the strategic balance. It's accurate enough to hit a city, which would provoke a tremendous response that is unthinkable for the CCP. But not accurate enough to act as a first strike weapon that can target an ICBM in it's silos (needing ~50-100m CEP for that). Getting around the US's ballistic missile shield, which is designed for NK, is a non-starter as they have SLBM and sufficient ICBM capabilities to saturate it and provide for MAD, besides their stated doctrine is a second-use only policy (although they would likely throw that out the fucking window if they went after Taiwan and things went crazy). As a nuclear weapon, it's just destabilizing, unless they intend to drastically increase the accuracy and use this as some sort of kinetic weapon system. But even then it's dangerous because a launch could be seen as potentially nuclear. I just don't see the point of this other than escalating defense development with the West.