My main argument against is that it is an out of context threat in the story, which doesn't make much sense at this point.
Also it would require the shards described to have access to a special power which is unique to them that is outside of their theme which doesn't make sense.
I would consider the idea it is frost to be more likely than another shard as the shards are so large, they influence the environment around them to the degree they cannot hide themselves.
Why if another shard is involved has there not been more signs of their influence, more than a strange moon, especially if you're insisting they were around long enough for them to have influenced Nohadon too?
Why have the other shards not been able to find him so close by?
Also how exactly was she caught off guard? All the information we had was she fled once retribution came into existence, we know future sight works on possibilities, she probably at most knew retribution was possible and could have waited ready in case it didn't.
Cultivation works as an intent for Nohadon equal or better than reason or Valor, and she has the means, motive and opportunity if she wanted to influence and grab his cognitive shadow on death