Who owns the IP rights to Nioh? I'm guessing KT still retains the IP ownership, similar to Ninja Gaiden even though they were published by Xbox.
So KT brought Ninja Gaiden to PlayStations via Sigma versions, they could do something similar to bring the whole trilogy to Xbox.
Keep in mind, Wo Long launched Day 1 on Gamepass, same with Ninja Gaiden 2 Black Remake, and of course Ninja Gaiden 4.
KT wants that Gamepass moneybag

. They could put 1 on Gamepass, while 2 remains on sale, then put 2 on Gamepass while 3 is released. Easy $100 million for KT between all three games on Gamepass.