I wouldn’t really want to rely on a single, relatively thin, flat plate of anything to maintain rigidity and positional accuracy over the size of the XXL baseboard & spoilboard. A nice chunk of aluminium as the top plate for a structure of, say, aluminium extrusion beams underneath providing the rigidity and adjustable levelling feet under that, now you’ve got a base.
On big mills the table is generally a thick steel casting with sufficient depth to provide rigidity and stability.
Since bolting down the baseboard of my XXL it’s a different machine. I deliberately included a layer of rubber as a material with a high loss-coefficient so that any vibrations that do occur get damped out quickly.