“the response was unusual, so obviously it can only mean what I want it to mean!”
No, it’s not the most troop deaths in one day. What makes it unique is that these soldiers weren’t killed on the front, in combat. Russia’s defenses failed to protect soldiers sleeping in their barracks.
That drives home something I was thinking when I first read the OP - The American barracks in Beirut in the Reagan years. I believe it was 241 killed. Yes, it was:
https://foreignpolicy.com/2014/02/07/when-reagan-cut-and-run/
Ukraine’s defense is failing to keep its electric grid viable.