So yeah, confirmed: US Plat AMEX no good for the Platinum VIP (right side) of the lounge. You MUST have the Credomatic version of the card to get at the good stuff.
However just show your regular US-issued Plat Amex and ID, and you're set for the SantaMaria VIP (left, aka. lame side) lounge.
Only a couple things I'd add to ariel_sjo's excellent ^ summary from above, re: the left (Santamaria) side:
1) The vegetables + Ranch sauce were nasty, looked like a week old, dried up, brown. Tamales were OK, brownies/cookies were fine, the salmon dip + crackers was passable, the weird cream cheese tortilla wrap thing looked nasty and all of these items were being kept at room temperature for god knows how long. (Food safety? What's that?
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2) The staff was very young and not acting in a particularly mature (much less professional) manner. I suspect the grown ups are running things on the other side. At least I'd sure hope so.
3) Menu items vastly overpriced and... food safety. Nope! You might actually do better on the plane for that one... (AS Cheese Plate, come to papa!
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4) LOUD place. Lots of hardscape surfaces and the staff seem to enjoy banging things around. By the time it was half-full, it was noisy enough to be really distracting and unpleasant.