It depends on the time/effort of the GA. If they don't start working until boarding time, or have other issues to deal with, sometimes those without seat assignments are left behind (regardless of the IDB list). The last thing a GA will do at T-10 min is to offload an already boarded passenger. If they don't have time to settle it pre-boarding, you may be the one denied boarding. Shouldn't be this way, but I've seen it happen too many times, sometimes excusable, sometimes with no CS line and the GAs chatting too much with each other prior to boarding. I don't think UA has any strict requirements here.