Yes yes i know Alliance/Horde players with certain heroes have temporarily united for a raids at least once or twice every expansion.
But a fully fledged cross faction guild functionality is huge departure from that, now its official that guilds and Azeroth "Champions" are now united.
Coming off of BFA this is a huge stretch without the necessary lore building blocks.
Even Blizzard doesn't pretend that everything is squared off and boxes ticked so... whats the story? Why are the faction restrictions being removed storywise? What happened for this monumental change? What diplomatic or threat forced this union?