As mentioned in this post from the SPG forum when you mentioned similar, sounds more like a miscommunication.

http://www.flyertalk.com/forum/27969013-post772.html

When the merger was announced in September, how the LT status 2018 onward would be handled wasn't addressed. If you're LT Plat w/ SPG that makes you Plat with Marriott this year.

There are reports in the SPG forum that Marriott is mistakenly sending out Gold cards rather than Plat cards to SPG Plats, but Lurker said to give Marriott time to get it sorted, or pass along your particulars to the Lurkers & they'll pass it along to the appropriate Marriott folk.

Cheers.