The giants are not stone. It’s just that they’ve been asleep for so long that stone and dirt has piled/formed on top of them, which gives you an idea of just how old Odin and the norse gods are.
Don’t forget, this whole “ToHP” story is being drawn by the amazing TL Collins!
TL Collins is the artist and creator of the web-comic known as Bullfinch. A comic known for it’s bizarre mixture of magic, monsters, fantasy, and squirrels thrown into a large blender and pulped into a fine paste. He has collaborated on several other comic works like Superfogeys, Th3rdworld’s Creature Feature, and Daniel Boom: AKA Loudboy.