He is therewith telling Erdogan to leave the Kurds alone. He can’t threaten Erdogan because Turkiye is a formal ally but it would not do to let Erdogan overrun our real ally in the area. This seems a quintessential Trump move.
I have no doubt it was proposed by Mattis and his staff, and approved by Trump. He a one sharp, well informed cookie, who knows his history and the players, so I’m sure his proposal and the reasons for it made sense to the big guy, and got green lighted for that reason.