Your not understanding. When ragnick was with red bull, he picked a few of the assistant coaches regardless on who was the head coach. If ragnick takes a front office role with man u, armas will most likely stay because he would have the support of ragnick.
They brought in Armas because recruiting assistants who understand the philisophy within a few weeks midseason during a pandemic is borderline impossible. Rangnick essentially admitted as much to the media. That's all there is to it. He'll be gone in the summer.