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Correct. I was getting the same quirk on their website offering to upgrade my Delta SkyMiles Platinum Card ($195 AF) to the Delta Reserve ($450 AF) almost immediately, even though it's not possible in the first year (will be doing it in January, a few months after the 1 year mark, in hopes that there's a MQM bonus that'll help with status for 2019/2020).
Usually with Amex charge cards, it only makes sense to upgrade if you have an aged card that would be better as a Platinum, you don't mind a lower once-in-a-lifetime bonus, and/or you don't want to open a new account. In my case, I upgraded my Green to Platinum that I'd had for well over a decade but was otherwise essentially useless. Since your card is so new anyway, I'd certainly just wait for a 100,000 MR SUB to appear again and app for a new Platinum account.