According to what? To me "boarding time" doesn't mean anything other than the time they plan to begin boarding. Is there actually a policy that passengers must

According to what? To me "boarding time" doesn't mean anything other than the time they plan to begin boarding. Is there actually a policy that passengers must be present by that time? What the parent quoted seems to be United's actual policy, i.e. that they'll accept any (domestic) passengers who arrive 15+ minutes before departure.