"Footless bird" means to keep moving forward and never stop.
Mana tells Allen, "Don't stop, keep going."
Almost throughout the entire exorcism.
The rabbi also mentioned in the article, "Because the Shuweng people are birds without feet, they must keep flying and never stop. Emotions are too heavy, and once they are carried on their backs, they cannot move forward."
Exorcists are all birds without feet. They keep fighting to prevent others from experiencing the same pain as themselves. The day they stop is the day they die.