Doesnt change the fact that it was an assumption that she dies at 50%. Shes not the first boss to do so. Archimonde in theory had 10% more health on mythic % wise compared to his normal and heroic counterparts.
Also we were already aware of the encounter having a different threshold for Mythic compared to Normal and Heroic. Its echos fault for not asking questions or reading the patch notes.
The race is created by the community. Not Blizzard. They dont have to announce anything regarding the race. And also it was announced.
There are EU players playing in Limit and vise versa. Also Limit streamed their Eternal palace race from London so thats not really an argument. If they deem it an issue then they could easily cover it.