Both factions were helping the nightbourne but they were having more positive interactions with the blood elves you can see play out during the story segment, even asking the blood elf leader if it was worth it to join the horde and even sympathizing with the mana addiction.
Also when the siege was over the night elves left and the blood elves stayed, this is talked about.
So no, this isnt remotely related to the sethrak in the slightest and the lore doesnt support your claims.
Nightbourne were shared content with dialogue completely positive with the blood elves with mixed dialogue from the night elves.
Sethrak is purely horde content and the horde is what helps them liberate their god and their people, alliance does nothing meaningful for them and they werent shared content, just horde content.
Doesnt sound like a partnership that would even remotely lean alliance, they dont even share the same beliefs.
This horde leaning wasnt inserted with the concept of AR’s, it was there at the start with the developing story.