If that's the case and the SAR experts know that and the oil on the surface can't possible be from the plane, crashed or submerged, then why are these experts taking note of this oil slick and sending 2 liters of it it in for identification???
I think that the answer to this is obvious... they don’t have any better leads.
We’re all speculating here based on a handful of facts. I tend to favor the theory that the crew & passengers were overcome by smoke/rapid-depressurization possibly while dealing with a progressive loss of power (perhaps of their own doing while isolating a fire).
But your guess is as good as mine.
Where I differ is on the possibility of a soft landing in those waters. I just don’t see it. That’s some big ocean out there. Rough waters are the rule.