As an XJ pilot, it is first come first serve. There is no other policy. Plus there is no authority given for another pilot to demand anything from a coworker.
I have got on before very senior captains just because I was there first.
I seem to recall something about arriving as a group with not enough seats, DOH would be used.
When jumpseating, I always wrote the time I got there on the form. Sorry for the hassel.
It is unfortunate that this other person acted this way. If the mainline captain follows through with his j/s coordinator, it could cause trouble for all of us.
Bottom line, you were right and have nothing to worry about.