The fact that the Celestials are susceptible to The Horde doesn't seem like that much of a stretch.
Any being can succumb to an infection or virus. Especially one that they haven't had the time to study.
Black Panther and Carol had the time and means to devise a way to fight them.
And whether Tony's Godkiller Armor can do what he built it for remains to be seen.
Because his books consistently sell. If people buy your work, you're going to keep getting jobs.