Those negotiations probably took place other the last month or more. The Syrian government will happily cooperate with SDF to close the Turkish border against Turkey supplying/trading with the terrorists including Daesh.I wonder if the Syrian government retakes all the "rebel" territory surrounding/in Aleppo whether it would negotiate for the "SDF" pocket or fight for it, or perhaps even leave it alone.
See this Al Monitor article ( Al Monitor is US based ) about the hole Ankara has dug for itself: