I think they meant that in any widely used Unix system today, there are a loooot of things which are not files, so the design is not honest.
The most common example of soemthing almost all programs interact with universally is BSD sockets. In Plan9, which goes out of its way to follow this everything is a file philosophy, TCP/UDP connections are actually represented as files. You open a file and write something to it to create the connection, and then you read or write to other files to read the streams, and you write again to the control file to close the connection. On the server side, you similarly write to a control file to start accepting packets, and monitor a directory to check for new connections, and so on.
Note that "file" here has a pretty clear definition: anything that can be interacted with using strictly the C FILE api - open()/read()/write()/close().
The most common example of soemthing almost all programs interact with universally is BSD sockets. In Plan9, which goes out of its way to follow this everything is a file philosophy, TCP/UDP connections are actually represented as files. You open a file and write something to it to create the connection, and then you read or write to other files to read the streams, and you write again to the control file to close the connection. On the server side, you similarly write to a control file to start accepting packets, and monitor a directory to check for new connections, and so on.
Note that "file" here has a pretty clear definition: anything that can be interacted with using strictly the C FILE api - open()/read()/write()/close().