This is another long-running myth.
Thorn was installed as a caretaker manager - a natural course of events after a sacking. Fans enjoyed his initial results and football, liked Thorn the man, called him The Godfather, wanted him to be the permanent manager. SISU obliged.
There is nothing to suggest he was only given the job full-time because he was cheap. Managers do not require a transfer fee and are on lower wages than any player; it wouldn't require much financial shuffling to bring in an outside man.