The technical discussions about mask variants are very interesting but I think that they are obscuring a key concern: that of the "panic" expressed from the flight deck.
No doubt the call across the PA was in line with procedures and there was no panic from the pilots, but it does seem to have been perceived as such by some if not all of the passengers. I could certainly believe that there would have been an abruptness or a seriousness of tone which would not be present in Capt Speaking's regular "cruising at 35,000' " announcement which would have been picked up by passengers and caused the more nervous to become genuinely concerned about.
Now obviously if you have just put the aircraft into a rapid descent due to pressurisation concerns the pax will notice and you can afford a tone of urgency but with no other indications it could be possible to adjust tone or wording to make the important message appear routine.
Passenger worry clearly isn't the overriding factor in this situation (that would be having the crew breathe and function normally) but I would far sooner read headlines like "Passengers unaware while pilots flew on oxygen" than some of the guff which we have seen with this incident.