Manager A - Inherits a team at the bottom end of the championship with a chance of being relegated. He manages, develops, coaches and improves a squad of players that were initially league one to lower championship standard on initially a very limited budget to create a team capable of being promoted and then stay in the premiership.
Manager B - Takes his team down into the championship. Has international players such as Shelvey, MBemba, Mitrovic at his disposal. Perez who had just had a very good season in the premiership, players easily good enough for the championship such as Colback, Dummett, Lasceles, Haidara, Anita AND has 60 million to spend on whoever he wants. Manager B get the team back to where they were when he took them over and also did well to keep them up
You are absolutely right when you say the two achievements are nowhere near comparable.