Oxygen can be supplied in both ways, so it is correct.

Remember the QANTAS 747 where an oxygen bottle exploded, ripping a hole in the side of the plane, forward of the wing? That was an oxygen bottle which is part of the gaseous ring main system used to supply emergency oxygen to the cabin, which are stowed in the hold of the aircraft, in the wall structure behind the panels of the cargo hold side walls.

We have a mix of systems across the fleet, both the ring main system, and the chemical generator system, both of which were fitted at different periods by the manufacturers.

I believe there is also an element of regulatory instruction on this subject, where as on our 777 aircraft we have a gaseous ring main system, in the US, the FAA dictate that chemical generators must be used, so a 777 with AA or UA would be different to ours.

These ring main systems are also what you will see crew using in the event that a passenger is required to use oxygen for medical reasons during the flight, and we have a process to go through with the flight crew to switch the system on, to ensure that it does not deploy all of the emergency oxygen masks throughout the cabin.