You are correct, American Airlines AAdvantage has never been a Membership Rewards transfer partner.
Go to:
http://membershiprewards.com
LOGIN
Next to
Available Points do you see
Available Points Advance 15,000?
If you click [Travel] do you see Transfer Points as an option?
If the Points Advance feature is offering you 15,000 points and you still have any Transfer Points award options I would say your account is still coded as if you were a Gold cardholder, but no telling how long before it will revert to Express.