From the last edition of 40K there was an alliance chart which dictated which armies could ally with each other, some of the factions like Orks and Dark Eldar would only side if the end was nigh. Specifically the term which described their alliance from the book was "Come the Apocalypse". I would describe a Tyranid invasion an Apocalypse. I could realistically see at a stretch the 4 factions in game temporarily fighting together. Weirder shit has happened in the lore, and I'm not talking just about now-redacted codexes. Edit: I know it's fairly tough to justify (Don't wanna ruin that deep Arkhona EC lore right?) but getting into PvE is borderline impossible in the game's current state. Allowing all factions to team up would fix this.