Previous experience has show, and we aren't the only ones if you read the links to the above, that there is more often then not confusion when the horn sounds. Valuable time has been squandered while crews faffed about. The idea is to get the masks on and then problem solve/deliberate.
The message after Helios was horn on the ground is config and airborne was cabin alt. Whilst we have established in the spoiler thread that there is a rare exception to that, there is a chance that it could confuse what should be a simple recognition and so is ignored.
the whole aircraft is a jig saw of later added pieces.
I think it's a very clear admission from Boeing that the pressurization system is a Human factors trap and from a time before ergonomics featured in system design, coupled with a MWS which is a relic and very limited in scope.