Originally Posted by
Twilight Cultist
There are a few reasons for deviation of the general 5 guild rule.
If a boss takes an unusual amount of time for 5 guilds to kill it (like Archimonde), the guild may release it when they feel enough time has gone by.
If there are lower end guilds (~top 15) who are on and start streaming their progress, some guilds may release their kill videos.
Also it is up to the individual guild who gets the world first. If a Chinese guild, for example, got World First Mythic Gul'dan, they probably wouldn't honor the 5 guild rule.
In the end, it isn't a hard rule, but generally, it is honored by the typical top guilds. Exorsus will likely not release their video until another guild kills Gul'dan or another 1-2 weeks go by without another kill.