Going by the oneworld rules, you only have lounge access when your inbound
long-haul business class flight connects to a
short-haul or domestic flight. The definition of long-haul seems to be 5 hours or more.
Therefore, it would depend on what flight you're taking in/out of Singapore as to whether you'd be granted access based on the connecting rule. Strictly following the rules, muppet should have had access, and exLXCXFF should not have.
Likely the lounge staff aren't well trained on this scenario as it's possibly not too common.
Connecting between oneworld marketed and operated flights:
- First and Business Class customers connecting on the same day of travel, or before 6am the following day, can access the lounge when travelling between an international long haul (a oneworld international long haul flight is defined as an international flight marketed and operated by any oneworld carrier with a scheduled flight time longer than 5 hours) and an international short haul or domestic flight (and vice-versa).
- Lounge access will be determined on the international long haul ticketed flight (either First of Business Class) regardless of the ticketed class of travel on the international short haul or domestic flight.
- You must be prepared to show your boarding pass or itinerary showing travel in First or Business class on the international long haul flight, in order to access the lounge before your international short haul or domestic flight.