I received the same "welcome to Diamond" message here in Europe on vacation. PM in mid 2016 with approx 26-27K rollovers (101-102K ending in 2016, with both both 15K reserve goals hit).
At first I thought it was an error as I just got Alaskan Air matching status before leaving. I also thought, maybe, just maybe, the complete and thoughtful surveys I completed made some difference... nah.
My glitch appears to be a repost of 15K MQMs for the $30K thresh on 12/31/16, and then 34K MQM's rollover on 1/1/17. That 34K is close to the total MQM's earned so far in 2017. On my iPhone, I should 34.6K with 40.7K to Plat, while on the computer I show 64.6K with 10K to Plat. None of the numbers completely true up.
I would assume AMEX or DL will reverse this and we'll get an apology letter come Monday.