"Kernels can use RAM too...the limit is 16EiB."
If kernels use motherboard RAM, they call the bottom half of the space: the lower 8EiB. The top half is reserved for, like I said, memory mapping and such. Apart from internal device memory (like GPU RAM), the bulk of the top half is intended to be "virtual" memory and not actually RAM sitting somewhere.
So the limit of motherboard RAM is 8 EiB.