(mostly joking) Lore, schmore! Who here cares about LORE (like I said, joking)?!? T'au didn't have Rail Rifles before Fire Warrior created them, Nobody beyond Telion had a Stalker bolter and now god and everybody has them! Just wave the magic wand the community knows you have and make it happen! Lol.
Dawn of war had allies in chaos and orks and then eldar and imperium but I guess GW have tightened the noose since then. Of course these allies didn't last long but they where part of the 1st games main story and I'm sure desperate allies against nids isn't too much of a stretch of the lore. (Looking at you blood angels and necrons)
Until the words "Training simulation" are added that is But don't' orks fight eachother all the time?
The biggest version of this kind of factional mix is a mobile game, which allows you to collect characters, across all factions and all eras, to fight in mixed teams, as a tabletop game, in the Star Wars universe. Star Wars Galaxy of Heroes. Inception basically. So yeah, if we say ... Arkhona really is an Armageddon battlefield and use the Alliance chart, detachments and all sorts of similar things, I'd piss off a lot of designers again.
Relatively low population combined with a very low percentage of the population we have even participating in PvE means it makes sense to allow mixed faction PvE even if it does completely break the lore.
