The report states
F-16s from Indiana National Guard[/I] intercepted the plane and followed it for about an hour until it crashed.
Is the most important line
Allowing time for the problem to develop, for ATC to notice the pilot not responding, ATC to then decide they have an emergency and ask for fighters, Fighters to scramble, get there and observe for an hour ... sounds like more than enough to run out of fuel. The A/P would then wind in a fair amount of trim before it disengaged and the aircraft would develop a spiral at modest speed above stall.
This clearly lasted long enough to identify where it was likely to crash and implement the 'get everyone inside' strategy.
All very consistent with an O2 failure at altitude while on autopilot.