Kevin Drum, a lonely lefty on this topic, pointed the way to the answer yesterday – the key is the meaning of “bullion coin” in the controversial language of the law:

(k) The Secretary may mint and issue platinum bullion coins 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.

People like Prof. Tribe are focusing on the Treasury Secretary’s discretion in setting denominations without worrying that the phrase “bullion coin” might actually have meaning.