That block is why 9-1-6-a-4 was added. The way the rule was initially written this year, that would have been a legal block, but that block has never (I think!) been legal and the committee corrected it.
I will agree that the block fits the definition above, and the flag would likely be supported by every conference coordinator of officials. I would further offer that the direction of the block is absolutely consistent with what would be termed a crack-back block. However, I can also see an opportunity to pass on the call if it is deemed that the defender was able to see the block coming and react appropriately. I understand that 10-2 positioning does not make a low crack-back legal, however few illegal crack-back blocks are as easy for the defender to see develop as the one on this video.
The intent of the rule (as described in Irving by Rogers Redding) is to make illegal a low block that is unlikely to be seen in time to react. It is not intended to punish every low block unobserved by the defender, but simply the vast majority of them -- hence the 10-2 standard. In the block on this video, the defender had far more time to react than is often afforded with a perfectly legal BBW.