While I agree, but if the decompression happened
really fast, the crew would have 15-20 seconds to react before losing consciousness, especially if both crew members were not wearing oxygen masks at the time the rapid decompression started. As such, I'm still sticking to the theory things happened so fast that while the crew was able to start an emergency descent sequence, they passed out with the plane only partly tilted down for an emergency descent. This may explain why the plane made what appears to be a "normal" descent straight into the ground in a
controlled flight into terrain crash.
We should find out more once both the cockpit voice recorder and flight data recorder data are fully recovered and analyzed.
Putting on O2 masks immediately is so second-nature that is likely not it. We do it all the time, in normal operations, and for a rapid decompression it is the first thing, every time. It takes 2 seconds with modern inflating masks.
You do not start the descent first, you put on O2 first. Always.