[The Squatter of Amon Rûdh]

I don't think we can say that with any certainty. After all, Gandalf doesn't normally anthropomorphise natural phenomena; and in any case Bilbo actually sees the Giants:

When he peeped out in the lightning-flashes, he saw that across the valley the stone-giants were out, and were hurling rocks one another for a game, and catching them, and tossing them down into the darkness where they smashed among the trees far below, or splintered into little bits with a bang.