The differences may be significant between the known AShBM like DF-21 and DF-26 boosted ones and that of the supposed YJ-21 ship launched ones. If YJ-21 is 055 carry-able, it probably has much shorter range than those MRBM and IRBM ranged AShBMs. The flight trajectory is probably quite different and uses a completely different approach - shockwave surfing WELL within the atmosphere.
The DF-26 and DF-21 launched payloads are probably much more conventional in flight paths with a different set of guidance principles and terminal phase maneuvering. These ballistic missile and hypersonic glide missiles are just two higher tier anti-surface weapons still shrouded in some secrecy although we know AShBM have been in service with PLA for at least a decade now. The HGV weapon/s are newer but if one hypersonic vehicle itself was shown in full back in October 2019, it only goes to prove there are plenty more modern and possibly superior anti-surface weapons already well in service. The CCP usually shows decades old stuff, they've moved towards showing years old stuff with greater transparency but rest assured everything displayed can be considered known by US intelligence.
YJ-21 is probably an HGV anti-ship missile. It is not comparable to AShBM. At the end of the day, all of this is expected from China by now. If we've known about AShBM since circa 2010, and we've known about HGVs and powered HGVs for years with successful flight tests confirmed by the US since 2006, then HGVs filling anti-ship roles is bound to happen. In fact they've had at least 15 years to make it so. Type 055 just happen to have some large enough VLS cells to carry YJ-21 when land and possibly air launched versions could have preceded ship launched versions.
Digital spectrum warfare is still kept far more secret than missiles. Both the US and China completely hide their EM wares. The risk with such network dependent high tier anti-surface missiles is exposure to electronic disruptions. Even if they knock out or jam some guidance chain, these missiles may be entirely useless! PLAN need to field many redundancies and keep those secret! YJ-12 and YJ-18 are probably not going to be enough against the USN.