The value obviously has nothing to do with the material choice, so why don't they just print it out of PLA?
The Secretary may mint and issue bullion and proof platinum coins in accordance with such specifications, designs, varieties, quantities, denominations, and inscriptions as the Secretary, in the Secretary's discretion, may prescribe from time to time.
Sorry, was your question rhetorical?
My best faith reading of the original comment was that it decries the arbitrary nature of platinum - if we are calling it a $trillion, why not make it out of anything?
But the debt ceiling is itself a legal construction. People call it a "loophole" because of episodes of The Simpsons or whatever but the Treasury is explicitly given permission to issue coins of arbitrary value. The debt ceiling is itself a legal construction.
The sovereign citizens and anti- Federal Reserve folks had fun with the idea in the 90s too.