A tun/tap device is a way to write a user space network driver for a piece of hardware to make it look like a network device to the OS. ie: you could make a serial serial cable look like a network device.

What this blog is asking for is a different way to access network resources so instead of calling a function like socket(some_ip, some_port, SOME_PROTOCOL); (which is a gross over simplification for the typical way it is done) he wants to be able to say open("/net/some_ip/protocol/some_port", "rw");