I think it is mix and match based on who is available on any given night between him and Robertson. Neither inspires a great deal of confidence right now imo as both have been just OK lately. Betances loaded the bases before getting out of it tonight.
Robertson hasn't given up in run in August or September with 23 ks in 14.2 innings. Your confidence aside, and how the Yankees decide to use them, Robertson has been anything but just "OK" since the All-Star break. That being said, since the calendar has flipped to September, Bentances has gotten all the saves and been deployed in situations usually reserved for closer. As of their most recent post, Closermonkey still has as a committee, but has flipped Betances back to the "lead" in the pen for saves.