One comment in that thread[1] gives a full explanation of what such a Raspberry Pi device hooked up to the router can do: forward all the network traffic, replace router's stock firmware with its own, install software on the network connected devices via known vulnerabilities, spoof websites by acting as custom DNS server. In my opinion, it looks like "a Pi-hole[2], but for phishing".
[1] https://www.reddit.com/r/whatisthisthing/comments/9ixdh9/fou...
[2] https://pi-hole.net/