Claim? By the OP's description of events, it would seem that that isn't just a potential "claim" by UA but in fact was what actually happened.

I don't think it's IDB. One requirement of being an IDB is that you hold a confirmed reservation for the flight. The OP didn't. The fact that they got a boarding pass means that they were a cleared standby passenger, not that they became a confirmed passenger once the BP was issued. You will notice that the monitors, website, and app will all show such a passenger as a "cleared standby" passenger - that is, they don't lose the distinction of being standby once the BP is issued.