The S-300V/SA-12 Giant is an ABM with fins on the upper stage and a very large booster.
The SM-3 series follows an incremental design process where the missile body and boosters for Block I is recycled from the SM-2 ER series. Had it been a clean slate design as a PLAN ABM would be, the size ratio of the booster and upper stages wouldn’t necessarily be what it is today.
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I didn't say that having fins on the upper stage = unable to be ABM. After all, I directly mentioned SM-3 in the same post, which has fins on its upper stage after its booster.
I was addressing the comparison to THAAD specifically, which alexander had made based on the images, which the YJ-21 one did not show the fins in the given frame.
As for the YJ-21 itself, to me it looks like the first stage/booster stage is as long, if not a bit longer than the upper stage, which while not incompatible with an ABM design, is slightly deviating from the norm. THAAD adopts such a configuration, but its upper stage is also finless.
Overall, my belief regarding this missile's identity, is that based on the timing of the emergence of this video clip, and the noise being made around it thus far, it seems more consistent with the expected YJ-XX ship launched AShBM/HGV, rather than the HQ-26 or HQ-19 ABM.