I didn't say it is attributed to Stephen Hawking. He simply recounted what he called an "old tale" of an elderly woman telling Bertrand Russell about the tower of turtles after Russell had given a lecture on astronomy. Actually, if I recall correctly, Hawking said "some say" it was Bertrand Russell who was giving the lecture.
Whether or not Bertrand Russell was the lecturer "That very simple sentence showed me, as I still think, the fallacy in the argument of the First Cause. If everything must have a cause, then God must have a cause. If there can be anything without a cause, it may just as well be the world as God, so that there cannot be any validity in that argument. It is exactly of the same nature as the Hindu's view, that the world rested upon an elephant and the elephant rested upon a tortoise; and when they said, "How about the tortoise?" the Indian said, "Suppose we change the subject."
Pratchett was born in 1948, so it seems his idea was inspiration, not invention. The idea seems to originate in Hindu mythology.
I haven't read any of Pratchett's novels, BTW. I'm currently on a Robert A. Heinlein streak.