I don’t want to put words in Ostrom’s mouth but my understanding is that this has to do with scaling.
The system works if it’s not too big or volatile, in other words, if everyone knows each other. You can easily imagine how the resource would be depleted if random people just showed up and started extracting it.
To scale, there’s a different rule (the last one) which calls for hierarchical organization. In case of the irrigation system there’s a group of people using the same distributory canal. That’s a closed group, no stragers allowed. However, for problems they can’t solve themselves they can coordinate with groups in control of other distributory canals.