Ok nevermind I guess I am dumb. You don't have to close the rifts but the game will tell you 28 times you need the mastery to close them.
The Skyscale was a time sink between living world seasons. Let’s not kid ourselves, the skyscale grind is a serious pain. Way back in May of 2019, when ArenaNet’s devs took us on a tour of the War Eternal episode and showed off the Skyscale, they glossed over the monumental task of unlocking the thing (note, the devs present were the ones who designed the mount’s mechanics, and they wHczko.