This FCOM Procedure comes from the AMM.
Normally you should not let the valve stay in its fully opened position (end stop). This prevents the valve from locking in the open position.
The AMM tells you during installation that:
...
To open the shutoff valves on the oxygen cylinders, do the steps that follow:
(a) Open the shutoff valve on the crew oxygen cylinder slowly and fully.
(b) Do not tighten the valve more than 25 inch-pounds (Hand tighten).
(c) Close the valve one-fourth of a turn.
(d) When you put the system back to its initial condition, you must use an inspection wire to keep the valve in this position.
...
And here is the problem. If you mess up the open/close direction, it is possible that you leave the system in a slightly open (one-fourth of a turn) position.
This can fill up the system (slowly) to the bottle pressure and the indicator in the cockpit shows a proper value.
But if you try to use O2, the flow rate is to low to supply a normal breathing person and you get this scenario described in the FCOM.
BTW: Firefighter open their bottles nearly the same way here in germany (half turn back)