The other example we have of thresholds is a Church.
- It's a public building.
- Everyone is theoretically invited.
- Maintenance/Electricity all apply.
The reason it gets a threshold status is because of "hallowed ground", and it's understood that there is some sort of divine protection going on.
While an eruv wouldn't have that divine aspect quite so strongly, there is certainly a religious component to it, which I would argue would make it more powerful than an your average "intrinsic" threshold created by a community/family who never gave any thought to it.
Another point in favor of an eruv is that in many cases the community is actively checking up on and religiously (pun intended) attending to it. It is common practice in the US to have an automated voice message and weekly e-mail telling people that the Eruv is "active". To me this feels like the constant maintenance described that is needed for focus items in the RPG.