You'll find endless past threads about getting out of line to go to the bathroom as well. You'll find that even for CMs once a person has exited the line they consider it an issue, this can vary by CMs however.
In a guest's situation it would be your (general your) opinion that it's not line cutting or it is line cutting.
As far as your already in line well that is exactly the excuse people use when the line is long so they send someone off to do something else in the meantime (get food or bathroom, etc) and then it's merely "I'm just meeting up with my family". In May we watched some kids get out of BTMRR, go get nachos and then meander their way back in line in front of us. By your definition they were in line already. Maybe you wouldn't have an issue, maybe others would and that's a whole 'nother conversation. We could haggle over "legit" or not here but it can become a quandary for guests to try and figure out if someone is authentic or not.
I agree with your last comment however I don't disagree with politely saying no, every guest has that ability to do so, whether they or the person they said no to feel they are both morally right or if the guest they said no to was against the Park's rules is different.