It is not stacked. It is multirank. Stacking means putting multiple layers on the same chip. They are already doing it for HBM. They will likely do it for other forms of DRAM in the future. Samsung reportedly will begin doing it in the 2030s:
I am not sure why they can already do stacking for HBM, but not GDDR and DDR. My guess is that it is cost related. I have heard that HBM reportedly costs 3 times more than DDR. Whatever they are doing to stack it now that is likely much more expensive than their planned 3D fabrication node.
https://www.tomshardware.com/pc-components/dram/samsung-outl...
I am not sure why they can already do stacking for HBM, but not GDDR and DDR. My guess is that it is cost related. I have heard that HBM reportedly costs 3 times more than DDR. Whatever they are doing to stack it now that is likely much more expensive than their planned 3D fabrication node.