Where do you see these requirements?
Good question. The terms I saw is for the international currency card:
https://www.americanexpress.com/icc/...five-star.html
While there are equivalent pages for platinums issued elsewhere, there doesn’t seem to to be one for US-issued platinums. The wording from inside the Amex app makes no mention of any requirement, except for the use of Amex CDP:
In addition to Gold Plus Rewards membership, when reserving a car with Hertz, use the Platinum Card from Amex Corporate Discount Code (CDP) to save on your rental, receive a 4-hour grace period on returns within the US, before an extra day charge is applied2, and get a one-car-class upgrade, subject to availability and limitations3.