That's what I was thinking too.

On FlightAware for a diverted flight it would normally show something like:

ANC - PVG
DTW - ANC
DTW - PVG DIVERTED

But instead it just shows DTW-ANC and ANC-PVG without there ever being a "diverted" remark. DL.com however shows it as "diverted".

I'm just speculating, but I believe it was a planned fuel stop because a) strong headwinds and/or b) there was a lot of cargo on, as this route is often payload optimized on the 744. Looking at the flight map they headed towards ANC right from the start, a normal flight path for DTW-PVG goes towards the North Pole (as DL583 today is doing).

Between the two flights, FlightAware believes the total for both legs was 7,703 miles and flying time of 14 hours and 14 minutes. DL says their 744 has a range of 7,365 miles. Seems to me like a planned stop to get more fuel and probably some additional crew members too.