My kid asks:
The Bavli, Eruvin 85 amud 2 just after the mishna, says that if a location is such that its resident's not being part of an eruv chatzeros doesn't prevent the eruv from taking effect for others, then that location is an unsuitable place for those others to place their eruv. It gives one exception: the gatehouse of a privately owned courtyard. Others can create an eruv without the gatehouse resident, but can nonetheless place their eruv in the gatehouse. But the courtyard is privately owned! Why would it need an eruv at all if the eruv doesn't include the gatehouse resident?