one silly question:

If the plane had boarded, should not all NRSA boarding last, to ensure no revenue passenger is missing. So if the pilot came after the NRSA, wasn't he too late (technically) to be on the flight.

As far as I know non-rev are waiting stand by at the gate until the flight is closed, and then and only then they board. This implies that the pilot arrived AFTER the flight was closed -- so he showed up to work.

It is not unusual that GA help out fellow FA/Pilots if they are commuting back and forth. I have also seen it that people / revenue passengers were waiting stand by on an oversold flight and a FA commuting to the international flight was let on and the passenger was put on the next flight. This is certainly not right -- but understandable from the airline's perspective : if the FA is not at the briefing fully dressed and ready in time she will be in huge trouble. So the GA helped her out, and the airline could send a long-haul flight out on time without calling a spare, which usually delays the flight.

This was on LH and not UA so it happens everywhere.

the question is HOW they do it.