It's not a finding of legal fault. It's a statement about whether or not things need to change in the aircraft design. The oxygen system (presumably, obviously I didn't design it) isn't inherently redundant. If it fails (because Stuff Always Breaks) there are existing procedures the pilots are presumably trained on. The article lists two: an emergency backup (probably a second tank in the cockpit?) and the old standby of "dive to lower altitudes so you don't need oxygen, dummy". The finding was that these options would have been sufficient if they were used, so the design of the aircraft isn't deficient.

Obviously the software bug should still be fixed, and probably pilot training needs to be updated.