A while ago a dude here strongly suggested that if a girl says she has a boyfriend, then we just have to back off and move on which I find to be completely misguiding.

Apparently no, it could be a real no, it could be a shit test or it could be just an excuse when she's still not sure what to do. My state tends to drop once the girl says she has a boyfriend because I was emotionally invested and outcome dependent which breaks my frame.

While we should recognize a straight no, sometimes a girl said she has a boyfriend but still stay in the set and gives out IOIs. The best thing to do according to my experience is to stay in the set, completely unfazed by her claim of having a boyfriend while remaining completely outcome independent.