If Warren sees Bernie's chances as DOA even with her endorsement, the progressive move might be making sure there is a progressive voice in the room in the administration of the inevitable winner. That was her rationale for not endorsing either Bernie or Hillary in 2016. She knew Bernie couldn't win, so she used her non-endorsement to extract progressive concessions from Hillary's team. Warren isn't in this for symbolic wins or principled defeats, she's in this to actually wield power to enact a progressive agenda.
If she had dropped out and endorsed Bernie before ST, Bernie would have won more states, thus increasing the chances of a progressive agenda actually being enacted.