No, all coast guards announcements are on 16 regardless of severity, because no one is monitoring other channels so any announcements on other channels would be like screaming in the void.

The initial call from help from the stranded boat was on 16 for sure, there is no other channel anyone would use for that.

The severity is instead communicated in the first part of the message with either « sécurité » for navigational hazards, « pan pan » for non life threatening events like this one, or « mayday » for life threatening events like man overboard, sinking etc.

The system is based on a single channel design specifically to avoid this situation. You only use other channels briefly, e.g. to talk to the dockmaster when coming into port, or to talk to another boat after having hailed it on 16. So it’s possible to miss a coast guard announcement, but they get repeated and in this case because they were the closest boat the coast guard must have tried to hail them multiple times.

My guess is that zuck yacht has a smaller boat attached to it, and they basically stayed on a different channel to communicate between the two for hours, never bothering to check 16. Which you aren’t supposed to do. Even then I would be surprised a $500M yacht can’t monitor two frequencies. I can on my little sailboat.