What is on Your Mind?

When the URL is corrected it leads to this:
Everything is a File
Unfortuatetly the content is not correct. Don't believe everything you read on the Internet.

The author or the article may have misunderstood the concept of "device-independent input/output" which is one of the fundamentals of predecessors or unix and of unix itself.

Indeed. Hopefully, not everything is a file under Unix. What is true is that many OS objects that aren't files on other operating systems, like devices (printers, disks, partitions, network interfaces, serial lines, ...), processes, virtual and physical memory, sockets, named pipes and the likes are files present in most Unix file systems hierarchies.

However, plenty of other OS objects, like partition tables, signals, threads, queues, statistics and others have no direct file equivalent.

In any case, as far as the operating system is concerned, a user (id) is just one of the many properties a process has.

I thought that a university website (which is furthermore on top of Google search results) would suffice as a valid learning resource.
I am however eager to learn, so if you have better sources I would love to read and learn about it.