How on earth could they rule out depressurisation and crew incapacitation? There is no basis to do that at this stage.
they would have had 8 minutes to have made ATC aware of a control problem yet made no contact?
The aircraft flew in a straight line surely any pilot faced with such a situation would try everything to gain control which would make it extremely unlikely that the aircraft would fly a straight line.
Every indication points to the pilots being incapacitated and the autopilot holding the aircraft on a heading and nothing to two alert pilots trying to control or regain control of an aircraft