I believe you’re information is correct. If you reboard, you are stuck on board. Or you can go explore and return in time for the new departure.
Earlier her this year in Sydney, we were all required to get off and were not allowed back on for about two hours. There was no place to go that had seating, restrooms etc. no waiting area in the terminal. I hunted for a place with some shade to wait it out until allowed back on board. It might have been nice to go see something of Sydney but I was using a mobility scooter so that wasn’t really an option. I parked in the shade and waited until the guards let us back on.
interestingly, I spoke to some passengers who had done a back to back on royal Caribbean a month or so, before my trip on Celebrity ???. They not only had to disembark and go through immigration, they had to take all their luggage off the ship and back on! I was very worried they would make us do the same but they didn’t.