There are many examples I have seen posted of Homeplugs connecting to a neighbours. I believe it is widespread.
Postings in the thread just checked to see if the desktop was hardwired or by wireless, so you are nit picking there. Physically there may well have been a cable connected but that wouldn't stop a wireless connection being made assuming it was nor Windows 8.
However your suggestion that Homeplugs can span into a separate adjacent property or flat is interesting. I've personally tried to do this using TP-Link 200 Mbps plugs between a flat above a shop. It didn't play ball. So tell me how to do it when you actually WANT the facility.