I can't imagine it's accurate enough to use for secure verification. I could see it's application for a shared entertainment system (ps4, netflix, etc) where identification is primarily for configuration purposes, not security.
Compared to crappy PIRs from companies like ADT, it is great stuff.
Still kinda evil though.
Also the paper notes this approach has fewer privacy concerns than other tracking systems, which is true to the degree your goal is tracking locations in a house. However if all the sudden we all ended up with tracking systems in our house that would be a privacy concern.
Can usually get in via custom order from anywhere that does window tinting.
a) There needs to be enough traffic happening on the network to have the resolution needed to differentiate between people.
b) The device needs to be able to see the traffic. This won't protect you from a device in the home that has this technology, but it will from people standing outside of the home (as the signal attenuates a lot outside of the home)
Conductive meshes work as well provided that the holes are small enough.
It's not doing internal positioning with Channel State Information (CSI), but there are folks that are . The approach in FIND is different  - FIND just uses RSSI+MAC information which works great on mobile devices and simple esp8266 chips and doesn't require code for the specific network interface. As far as I know you have to write some network card specific code to be able to use CSI.
Happy to answer any questions!
Edit: We're hiring DSP, ML and Growth people