Out of curiosity: Why would one choose the offered plastic plate over steel oder aluminium? I would expect it to be inferiour regarding for example flex and noise/sound.
Is it about aesthetics?
If I recall correctly, it has to do with manufacturing. The plates are laser-cut and not CNC machined. Aluminum in this case would require them to use CNC services, which would be costlier than manufacturing SS and plastic plates as far as time/process/supply of raw material goes.
Plastic is not necessarily inferior as a plate. It all depends on the build and feel you are trying to accomplish. There are a lot of very high-quality keyboards with polycarbonate/plastic plates or half-plates as well, and some enthusiasts prefer them.