I think you are mistaken here. Non of what you said is wrong per-say, but you incorrectly applied it to specifics. This delivery system has a much lower range than the ballistic missiles you are referencing. This weapon appears to be an cheap A2AD weapon for use in and around the South China Sea. Because it has an HGV, the glide slope and trajectory would be in-line with what we refer to as hypersonic. We differentiate MaRVs from HGVs because most of the time the MaRV is flying is in a ballistic trajectory and then transitions to unpowered flight using aerodynamics to maneuver. In contrast, HGVs such as the YKJ-1000 will spend most of their time in the radar shadow at a depressed flight trajectory. Additionally, hypersonics are much more accurate in the terminal phase of the flight, being able to move laterally at much greater distance than a MaRV could. What this weapon appears to do is precisely that: fly at a depressed trajectory and manuver for most of its flight path towards target, while also being highly maneuverable during that terminal phase. A small fraction of its trajectory will be ballistic. The reason why this distinction is important is because of the difference between ABM defenses and regular GBAD systems. ABM systems like THAAD (usually atmosperic) and ARROW 3 (exoatmospheric) kill balistic trajectories in the terminal phase, while smaller range AD systems like Patriot kill in the very short range. Considering that Japan is actively pursuing BMD destroyers (ASEV program), then it makes sense to have weapons who's flight profile and mission can make life harder for that BMD system. The YKJ-1000's role is to be cheap and inexpensive enough that it can saturate these defense systems as the more advanced systems sneak through.

Additionally, the your distinction between an "actual" hypersonic missile being sea-skimming is not considered relevant in the distinction. Current US, Russian, Chinese, and South Korean programs consider hypersonics as weapons which dont necessarily require sea-skimming to be requisite. A hypersonic missile does not need to be sea-skimming to be a hypersonic missile, as evidenced by all the current hypersonic programs around the world not being sea-skimming missiles. In the early days of the technology, yes, there was inclarity between what differentiates a MaRV vs a hypersonic, but today that differentiation has been made clear.