[brxbrx]

We don't know what the world of Dominions is made of. The sun is literally a floating disk,FFS.
Let's say that boron is the base of giant bones. In our world, there probably wouldn't be enough for several species of giants. But who's to say that the Dominions is the same? The reason we're short of boron here is because it's not formed by stellar nucleosis, unlike the far more common hydrogen and carbon. But in Dominions, we don't know how the world was created. Boron could be plentiful enough for it to be a part of a giant's skeletal system. It's certainly strong enough.