Its quite simple.

In an emergency the crew are expected to be in absolute optimum health - Tech Crew included. It is also an Occupational Health and Safety Hazard. That is how they differ to the passengers. Similarly, restaurant workplaces have staff toilets and customer toilets. This request is usually made by the Captain and may have been beyond the Pursers control.

As I mentioned also, Crew Control may have advised that soemthing could be going around in HKG. As I said an explanation from the Operating Crew probably could end all this speculation, im just trying to offer some "inside" explanation for it.

We used to block off a toilet for the crew on all BOM flights after passenger after passenger sh*t in the basin. It was decided it was not within safe work practices to have that standard of fascitlities.

I certainly wouldnt want a sick Captain onboard or to have to attend to a pilot incapacitation because he became ill during the flight and lost consciousness.