This report is surprising as only a few days ago the US Department of Defense declared that any Syrian, Russian, or Iranian action against ISIS in the part of Syria occupied by US troops was "unaccptable."  https://www.rt.com/news/446328-pentagon-syria-turkey-military/ 

In other words, the Pentagon declared that the part of Syria still in ISIS's hands was under US protection and was not to be attacked by Syria and its allies.

The question before us: Is Trump's reported order of withdrawal going to happen?

It is too soon to say. As I write no support for Trump's withdrawal order has been reported in the media. Even the official statement by the White House's own spokesperson, Sarah Sanders, is unclear: "We have started returning United States troops home as we transition to the next phase of this campaign." https://www.zerohedge.com/news/2018-12-19/drastic-reversal-us-prepares-full-withdrawal-forces-syria-immediately 

In other words, the campaign against Syria is not over, but "the next phase" apparently doesn't need US troops. What is the next phase? Until we know the answer, we do not know whether Washington's campaign against Assad is over.