Yes I am sure the researchers have no idea what a firewall is. And everyone is a network admin with their home routers...

Of course these researches used logic to determine when a firewall is in place. One possible way would be to look at a subnet as a whole, if neighboring IPs are responding you can make a reasonable guess that other IPs should respond if binded to another node. This is a sampling of 4 billion, so no, individual circumstances where this doesn't hold up won't make a difference.

Wait for the actual paper to come out during the conference. If your research with your home router shows this is an incorrect paper, you can call them out. After all this is what peer review is all about.