In Ineffa's weapon lore, Nibelung already left Teyvat even before HP arrived so they werent even responsible for Nibelung traveling outside and got corrupted by the Abyss.
Being conquered by HP probably the best option for Teyvat. If HP didnt arrive, corrupted Nibelung gonna destroyed entire Teyvat and the Dragons and Moon Sisters couldnt stop him. With HP around, Teyvat still survives for a while.