thats not what happened with me. I just went back to the tapu's area after the credits and faced it again.
you don't have to face the Elite four and defend your championship
Hope I Helped!!!
(oh and who would want to trade anything for tapu koko? each and every person can get it, no matter how many times you KO it. now if it was a Solgaleo for a Lunala then yeah thats understandable)