Hasn't the US line always been that they aren't sending weapons and equipment to Totally Not The PKK and instead Arab units in the SDF anyway? 
Pretty much. The YPG and PKK are not really distinguishable, because the YPG was formed by PKK members in Syria and includes a hell of a lot of people born in Turkey or trained in Turkey. American socialist Brace Belden, who fought in Syria for several years against ISIS, suggests that he was transported and partially trained directly by the PKK, and not a YPG affiliate.
In 2015 the US created the Syrian Democratic Forces to be something like a shell corporation -- a way for the government to fund PKK affiliates without violating any of our promises to Turkey. While there
are a lot of fully-Arab units in the SDF, coordination between these local units and the YPG (and possibly with hardened PKK veterans from Turkey) is very thick and the SDF makes an effort to introduce these groups to their libertarian socialist worldview.
By apparently acknowledging the inseparability of the YPG and PKK Trump is being unusually honest here, but only because he's trying to appease Turkish politicians who have genocidal ambitions toward Kurds.