I think it means that, even if you pass, if you're the last one to be left inside the instance or instance part/level/segment it will still end up in your loot.

Not really sure but, something like this may have happened to me while speedrunning WP. I passed on the Tonberry King loot (random DL weapon) but since other 3 people in my DF party dropped duty, instead of exiting via portal, and I was the last one inside WP the DL weapon ended in my loot. Regardless on me passing on it.

Still, CT lockout logic doesn't go past the "let's make the content last longer" as you can gear yourself better in Coil, where you can get more than one drop per weekly lockout for more than one job. Of course, either if you're lucky or your party passes on items in your favour. Still chuckling at the "used for vanity" comment on CT loot they made in the live letter.... Oh well.